DevTools: do not assign empty content to CSS UISourceCode in case of protocol error

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

 // Copyright 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.
 /**
  * @implements {Common.ContentProvider}
  * @unrestricted
  */
 SDK.CSSStyleSheetHeader = class {
   /**
    * @param {!SDK.CSSModel} cssModel
@@ skipping 111 matching lines @@
    */
   contentType() {
     return Common.resourceTypes.Stylesheet;
   }
 
   /**
    * @override
    * @return {!Promise<?string>}
    */
   requestContent() {
-    return /** @type {!Promise<?string>} */ (this._cssModel.getStyleSheetText(this.id));
+    return this._cssModel.getStyleSheetText(this.id);
   }
 
   /**
    * @override
    * @param {string} query
    * @param {boolean} caseSensitive
    * @param {boolean} isRegex
    * @param {function(!Array.<!Common.ContentProvider.SearchMatch>)} callback
    */
   searchInContent(query, caseSensitive, isRegex, callback) {
@@ skipping 62 matching lines @@
      */
     function performSearch(content) {
       var searchResults =
           content ? Common.ContentProvider.performSearchInContent(content, query, caseSensitive, isRegex) : [];
       callback(searchResults);
     }
 
     this.requestContent().then(performSearch);
   }
 };