Devtools: Use Event.deepPath as a function if available.

In crrev.com/1637813002 Event.deepPath changes from an attribute to a
function. Event.deepPath is the prefered method for retriving all
elements at point in an event. This change also adds a fallback to
Event.path for old front-ends targeting Event.deepPath as an attribute.

Review URL: https://codereview.chromium.org/1653213002

Cr-Commit-Position: refs/heads/master@{#374084}

front_end/devtools.js

@@ -1010,6 +1010,9 @@ function installBackwardsCompatibility()
     styleElement.type = "text/css";
     styleElement.textContent = "html /deep/ * { min-width: 0; min-height: 0; }";
     window.document.head.appendChild(styleElement);
+
+    // Support for legacy (<M49) frontends. Remove in M52.
+    Event.prototype.deepPath = undefined;
 }
 
 function windowLoaded()

front_end/ui/Tooltip.js

@@ -33,7 +33,7 @@ WebInspector.Tooltip.prototype = {
      */
     _mouseMove: function(event)
     {
-        var path = event.deepPath ? event.deepPath : event.path;
+        var path = event.path;
         if (!path || event.buttons !== 0)
             return;