DevTools: Fix visible whitespace for cm_modes

third_party/WebKit/Source/devtools/front_end/source_frame/SourcesTextEditor.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.
 
 /**
  * @constructor
  * @extends {WebInspector.CodeMirrorTextEditor}
  * @param {!WebInspector.SourcesTextEditorDelegate} delegate
  */
 WebInspector.SourcesTextEditor = function(delegate)
@@ skipping 467 matching lines @@
     {
         this._muteTextChangedEvent = true;
         this._setEditorIndentation(text.split("\n").slice(0, WebInspector.SourcesTextEditor.LinesToScanForIndentationGuessing));
         WebInspector.CodeMirrorTextEditor.prototype.setText.call(this, text);
         delete this._muteTextChangedEvent;
     },
 
     /**
      * @override
      * @param {string} mimeType
+     * @return {!Promise}
      */
     setMimeType: function(mimeType)
     {
         this._mimeType = mimeType;
-        WebInspector.CodeMirrorTextEditor.prototype.setMimeType.call(this, this._applyWhitespaceMimetype(mimeType));
+        return WebInspector.CodeMirrorTextEditor.prototype.setMimeType.call(this, mimeType)
+            .then(() => this._codeMirror.setOption("mode", this._applyWhitespaceMimetype(mimeType)));
     },
 
     _updateWhitespace: function()
     {
         if (this._mimeType)
             this.setMimeType(this._mimeType);
     },
 
     /**
      * @param {string} mimeType
@@ skipping 488 matching lines @@
         this._codeMirror.addOverlay(overlayMode);
         this._highlightDescriptor = {
             overlay: overlayMode,
             selectionStart: selectionStart
         };
     }
 }
 
 WebInspector.SourcesTextEditor.LinesToScanForIndentationGuessing = 1000;
 WebInspector.SourcesTextEditor.MaximumNumberOfWhitespacesPerSingleSpan = 16;