DevTools: Do not toggle line styles unless necessary in script editor.

Source/devtools/front_end/sources/JavaScriptSourceFrame.js

 /*
  * Copyright (C) 2011 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 756 matching lines @@
     _onSourceMappingChanged: function(event)
     {
         var data = /** @type {{target: !WebInspector.Target}} */ (event.data);
         this._updateScriptFile(data.target);
         this._updateLinesWithoutMappingHighlight();
     },
 
     _updateLinesWithoutMappingHighlight: function()
     {
         var linesCount = this.textEditor.linesCount;
-        for (var i = 0; i < linesCount; ++i)
-            this.textEditor.toggleLineClass(i, "cm-line-without-source-mapping", !WebInspector.debuggerWorkspaceBinding.uiLineHasMapping(this._uiSourceCode, i));
+        for (var i = 0; i < linesCount; ++i) {
+            var lineHasMapping = WebInspector.debuggerWorkspaceBinding.uiLineHasMapping(this._uiSourceCode, i);
+            if (!lineHasMapping)
+                this._hasLineWithoutMapping = true;
+            if (this._hasLineWithoutMapping)
+                this.textEditor.toggleLineClass(i, "cm-line-without-source-mapping", !lineHasMapping);
+        }
     },
 
     /**
      * @param {!WebInspector.Target} target
      */
     _updateScriptFile: function(target)
     {
         var oldScriptFile = this._scriptFileForTarget.get(target);
         var newScriptFile = WebInspector.debuggerWorkspaceBinding.scriptFile(this._uiSourceCode, target);
         this._scriptFileForTarget.remove(target);
@@ skipping 128 matching lines @@
         this._uiSourceCode.removeEventListener(WebInspector.UISourceCode.Events.WorkingCopyChanged, this._workingCopyChanged, this);
         this._uiSourceCode.removeEventListener(WebInspector.UISourceCode.Events.WorkingCopyCommitted, this._workingCopyCommitted, this);
         this._uiSourceCode.removeEventListener(WebInspector.UISourceCode.Events.TitleChanged, this._showBlackboxInfobarIfNeeded, this);
         WebInspector.settings.skipStackFramesPattern.removeChangeListener(this._showBlackboxInfobarIfNeeded, this);
         WebInspector.settings.skipContentScripts.removeChangeListener(this._showBlackboxInfobarIfNeeded, this);
         WebInspector.UISourceCodeFrame.prototype.dispose.call(this);
     },
 
     __proto__: WebInspector.UISourceCodeFrame.prototype
 }