[devtools] Replace usage of widget symbol by using the getter

This change enables the removal of widgetSymbol and replacing it
with a WeakMap.

R=tvanderlippe@chromium.org

Bug: chromium:1011811
Change-Id: I36434c734cc89df5b7c9d5eed4bf1a588d77b36f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532777Reviewed-by: Tim van der Lippe <tvanderlippe@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826741}

third_party/blink/web_tests/http/tests/devtools/help/release-note.js

@@ -35,9 +35,9 @@
   Help._showReleaseNoteIfNeeded();
 
   function onShowView(viewId, isUserGesture, viewPromise) {
-    viewPromise.then(() => {
+    viewPromise.then(async () => {
       var releaseNoteView = UI.viewManager.view('release-note');
-      var releaseNoteElement = releaseNoteView[UI.View.widgetSymbol].contentElement;
+      var releaseNoteElement = (await releaseNoteView.widget()).contentElement;
       TestRunner.addResult('Dumping release note text:');
       TestRunner.addResult(releaseNoteElement.innerText);
       TestRunner.addResult('Last version of release note seen should be updated:');