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

Source/devtools/front_end/source_frame/CodeMirrorTextEditor.js

 /*
  * Copyright (C) 2012 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 1052 matching lines @@
         delete this._executionLine;
     },
 
     /**
      * @param {number} lineNumber
      * @param {string} className
      * @param {boolean} toggled
      */
     toggleLineClass: function(lineNumber, className, toggled)
     {
+        if (this.hasLineClass(lineNumber, className) === toggled)
+            return;
         var lineHandle = this._codeMirror.getLineHandle(lineNumber);
         if (!lineHandle)
             return;
         if (toggled)
             this._codeMirror.addLineClass(lineHandle, "wrap", className);
         else
             this._codeMirror.removeLineClass(lineHandle, "wrap", className);
     },
 
     /**
      * @param {number} lineNumber
+     * @param {string} className
+     * @return {boolean}
+     */
+    hasLineClass: function(lineNumber, className)
+    {
+        var lineInfo = this._codeMirror.lineInfo(lineNumber);
+        var wrapClass = lineInfo.wrapClass || "";
+        var classNames = wrapClass.split(" ");
+        return classNames.indexOf(className) !== -1
+    },
+
+    /**
+     * @param {number} lineNumber
      * @param {!Element} element
      */
     addDecoration: function(lineNumber, element)
     {
         var widget = this._codeMirror.addLineWidget(lineNumber, element);
         this._elementToWidget.set(element, widget);
     },
 
     /**
      * @param {number} lineNumber
@@ skipping 1362 matching lines @@
     function tokenOverride(superToken, stream, state)
     {
         var token = superToken(stream, state);
         return token ? tokenPrefix + token.split(/ +/).join(" " + tokenPrefix) : token;
     }
 }
 
 WebInspector.CodeMirrorTextEditor._overrideModeWithPrefixedTokens("css", "css-");
 WebInspector.CodeMirrorTextEditor._overrideModeWithPrefixedTokens("javascript", "js-");
 WebInspector.CodeMirrorTextEditor._overrideModeWithPrefixedTokens("xml", "xml-");