DevTools: migrate from external Maps to symbols in the bindings objects.

third_party/WebKit/Source/devtools/front_end/css_tracker/CSSTrackerView.js

 // Copyright (c) 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 CSSTracker.CSSTrackerView = class extends UI.VBox {
   constructor() {
     super(true);
 
     this.registerRequiredCSS('css_tracker/cssTrackerView.css');
 
@@ skipping 377 matching lines @@
   /**
    * @override
    * @param {!Workspace.UISourceCode} uiSourceCode
    * @param {!TextEditor.CodeMirrorTextEditor} textEditor
    */
   decorate(uiSourceCode, textEditor) {
     var gutterType = 'CodeMirror-gutter-coverage';
 
     var decorations = uiSourceCode.decorationsForType(CSSTracker.CSSTrackerView.LineDecorator.type);
     textEditor.uninstallGutter(gutterType);
-    if (!decorations.size)
+    if (!decorations || !decorations.size)
       return;
 
     textEditor.installGutter(gutterType, false);
 
     for (var decoration of decorations) {
       for (var line = decoration.range().startLine; line <= decoration.range().endLine; ++line) {
         var element = createElementWithClass('div');
         if (decoration.data())
           element.className = 'text-editor-css-rule-used-marker';
         else
           element.className = 'text-editor-css-rule-unused-marker';
 
         textEditor.setGutterDecoration(line, gutterType, element);
       }
     }
   }
 };
 
 CSSTracker.CSSTrackerView.LineDecorator.type = 'coverage';