DevTools: introduce "Track CSS Usage" experiment

third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 427 matching lines @@
     return this._ensureOriginalStyleSheetText(styleSheetId)
         .then(() => this._agent.setKeyframeKey(styleSheetId, range, text, callback.bind(this)))
         .catchException(false);
   }
 
   startRuleUsageTracking() {
     this._agent.startRuleUsageTracking();
   }
 
   /**
-   * @return {!Promise<?Array<!WebInspector.CSSModel.RuleUsage>>}
+   * @return {!Promise<!Array<!WebInspector.CSSModel.RuleUsage>>}
    */
   ruleListPromise() {
     /**
      * @param {?string} error
      * @param {!Array<!Protocol.CSS.RuleUsage>=} ruleUsage
-     * @return {?Array<!WebInspector.CSSModel.RuleUsage>}
+     * @return {!Array<!WebInspector.CSSModel.RuleUsage>}
      */
     function usedRulesCallback(error, ruleUsage) {
       if (error || !ruleUsage)
-        return null;
+        return [];
 
       return ruleUsage.map(rule => ({range: rule.range, styleSheetId: rule.styleSheetId, wasUsed: rule.used}));
     }
 
     return this._agent.stopRuleUsageTracking(usedRulesCallback);
   }
 
   /**
    * @return {!Promise.<!Array.<!WebInspector.CSSMedia>>}
    */
@@ skipping 737 matching lines @@
 WebInspector.CSSModel.InlineStyleResult = class {
   /**
    * @param {?WebInspector.CSSStyleDeclaration} inlineStyle
    * @param {?WebInspector.CSSStyleDeclaration} attributesStyle
    */
   constructor(inlineStyle, attributesStyle) {
     this.inlineStyle = inlineStyle;
     this.attributesStyle = attributesStyle;
   }
 };