HTML/events/beforeunload - respect preventDefault().

An existing web-platform-test was enhanced to make sure
the preventDefault only (or combined) cases work.

Added many layout tests (due to the lack of
web-platform-tests APIs for handling beforeunload alert)
for most of the intricacies of beforeunload.

Bug: 866818
Change-Id: I35c27cee1f71a4e4d10331d3451e0a36b9db684d

html/browsers/browsing-the-web/unloading-documents/beforeunload-canceling-1.html

@@ -7,8 +7,12 @@ <h1>If this goes away, it navigated</h1>
 <script>
 "use strict";
 
-window.runTest = (t, { valueToReturn, expectCancelation, setReturnValue, expectedReturnValue }) => {
+window.runTest = (t, { valueToReturn, expectCancelation, setReturnValue, expectedReturnValue, cancel }) => {
   window.onbeforeunload = t.step_func(e => {
+    if (cancel) {
+      e.preventDefault();
+    }
+
     if (setReturnValue !== undefined) {
       e.returnValue = setReturnValue;
     }

html/browsers/browsing-the-web/unloading-documents/beforeunload-canceling.html

@@ -154,6 +154,37 @@
     expectCancelation: true,
     setReturnValue: "foo",
     expectedReturnValue: "foo"
+  },
+  {
+    expectCancelation: true,
+    cancel: true,
+    expectedReturnValue: ""
+  },
+  {
+    setReturnValue: "foo",
+    expectCancelation: true,
+    expectedReturnValue: "foo",
+    cancel: true
+  },
+  {
+    valueToReturn: "foo",
+    expectedReturnValue: "foo",
+    expectCancelation: true,
+    cancel: true
+  },
+  {
+    valueToReturn: "foo",
+    setReturnValue: "foo",
+    expectedReturnValue: "foo",
+    expectCancelation: true,
+    cancel: true
+  },
+  {
+    valueToReturn: true,
+    setReturnValue: "",
+    expectedReturnValue: "true",
+    expectCancelation: true,
+    cancel: true
   }
 ];
 
@@ -164,6 +195,11 @@
   const labelAboutReturnValue = testCase.setReturnValue === undefined ? "" :
     `; setting returnValue to ${testCase.setReturnValue}`;
 
+  const labelAboutCancel = testCase.cancel === undefined ? "" :
+    "; calling preventDefault()";
+
+  const suffixLabels = labelAboutReturnValue + labelAboutCancel;
+
   async_test(t => {
     const iframe = document.createElement("iframe");
     iframe.onload = t.step_func(() => {
@@ -174,7 +210,7 @@
 
     iframe.src = "beforeunload-canceling-1.html";
     document.body.appendChild(iframe);
-  }, `Returning ${testCase.valueToReturn} with a real iframe unloading${labelAboutReturnValue}`);
+  }, `Returning ${testCase.valueToReturn} with a real iframe unloading${suffixLabels}`);
 }
 
 runNextTest();