Ensure there is an active script while running JS jobs

"Fixes" tc39/ecma262#871, at least for HTML,
given that we have our own version of EnqueueJob. Important for #3117.

source

@@ -3014,6 +3014,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-enumerableownproperties">EnumerableOwnProperties</dfn> abstract operation</li>
      <li>The <dfn data-x="js-FunctionCreate" data-x-href="https://tc39.github.io/ecma262/#sec-functioncreate">FunctionCreate</dfn> abstract operation</li>
      <li>The <dfn data-x="js-Get" data-x-href="https://tc39.github.io/ecma262/#sec-get-o-p">Get</dfn> abstract operation</li>
+     <li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-getactivescriptormodule">GetActiveScriptOrModule</dfn> abstract operation</li>
      <li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-getfunctionrealm">GetFunctionRealm</dfn> abstract operation</li>
      <li>The <dfn data-x-href="https://tc39.github.io/ecma262/#sec-hasownproperty">HasOwnProperty</dfn> abstract operation</li>
      <li>The <dfn data-x="js-HostEnsureCanCompileStrings" data-x-href="https://tc39.github.io/ecma262/#sec-hostensurecancompilestrings">HostEnsureCanCompileStrings</dfn> abstract operation</li>
@@ -86186,6 +86187,16 @@ interface <dfn>NavigatorOnLine</dfn> {
   data-x="concept-script">script</span>. It has no additional <span data-x="struct
   item">items</span>.</p>
 
+  <p>The <dfn>active script</dfn> is determined by the following algorithm:</p>
+
+  <ol>
+   <li><p>Let <var>record</var> be <span>GetActiveScriptOrModule</span>().</p></li>
+
+   <li><p>If <var>record</var> is null, return null.</p></li>
+
+   <li><p>Return <var>record</var>.[[HostDefined]].</p></li>
+  </ol>
+
   <hr>
 
   <p>An <dfn data-export="">environment</dfn> is an object that identifies the settings of a
@@ -87933,6 +87944,17 @@ document.querySelector("button").addEventListener("click", bound);
 
    <li><p>Let <var>incumbent settings</var> be the <span>incumbent settings object</span>.</p></li>
 
+   <li><p>Let <var>active script</var> be the <span>active script</span>.</p></li>
+
+   <li><p>Assert: <var>active script</var> is not null, as jobs are only enqueued by the JavaScript
+   specification while a script is active.</p></li>
+
+   <li><p>Let <var>script execution context</var> be a new <span>JavaScript execution
+   context</span>, with its Function field set to null, its Realm field set to <var>active
+   script</var>'s <span>settings object</span>'s <span data-x="environment settings object's
+   Realm">Realm</span>, and its ScriptOrModule set to <var>active script</var>'s <span
+   data-x="concept-script-record">record</span>.</p></li>
+
    <li>
     <p><span>Queue a microtask</span>, on <var>job settings</var>'s <span>responsible event
     loop</span>, to perform the following steps:</p>
@@ -87941,13 +87963,36 @@ document.querySelector("button").addEventListener("click", bound);
      <li><p><span>Check if we can run script</span> with <var>job settings</var>. If this returns
      "do not run" then return.</p></li>
 
-     <li><p><span>Prepare to run script</span> with <var>job settings</var>.</p></li>
+     <li>
+      <p><span>Prepare to run script</span> with <var>job settings</var>.</p>
+
+      <p class="note">This affects the <span data-x="concept-entry-everything">entry</span> concept
+      while the job runs.</p>
+     </li>
+
+     <li>
+      <p><span>Prepare to run a callback</span> with <var>incumbent settings</var>.</p>
 
-     <li><p><span>Prepare to run a callback</span> with <var>incumbent settings</var>.</p></li>
+      <p class="note">This affects the <span data-x="concept-incumbent-everything">incumbent</span>
+      concept while the job runs.</p>
+     </li>
+
+     <li>
+      <p><span data-x="stack push">Push</span> <var>script execution context</var> onto the
+      <span>JavaScript execution context stack</span>.</p>
+
+      <p class="note">This affects the <span>active script</span> while the job runs, in cases like
+      <code data-x="">Promise.resolve("...").then(eval)</code> where there would otherwise be no
+      active script since <code data-x="">eval</code> is a built-in function that does not originate
+      from any particular <span data-x="concept-script">script</span>.</p>
+     </li>
 
      <li><p>Let <var>result</var> be the result of performing the abstract operation specified by
      <var>job</var>, using the elements of <var>arguments</var> as its arguments.</p></li>
 
+     <li><p><span data-x="stack pop">Pop</span> <var>script execution context</var> from the
+     <span>JavaScript execution context stack</span>.</p></li>
+
      <li><p><span>Clean up after running a callback</span> with <var>incumbent
      settings</var>.</p></li>