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

Source/devtools/front_end/source_frame/CodeMirrorTextEditor.js

Index: Source/devtools/front_end/source_frame/CodeMirrorTextEditor.js
diff --git a/Source/devtools/front_end/source_frame/CodeMirrorTextEditor.js b/Source/devtools/front_end/source_frame/CodeMirrorTextEditor.js
index b34e9ef98abc27f547ad7880d2b4285640b0c1cb..da3c3f7f290d9044aed0db9bb10bb538a45d0aae 100644
--- a/Source/devtools/front_end/source_frame/CodeMirrorTextEditor.js
+++ b/Source/devtools/front_end/source_frame/CodeMirrorTextEditor.js
@@ -1070,6 +1070,8 @@ WebInspector.CodeMirrorTextEditor.prototype = {
      */
     toggleLineClass: function(lineNumber, className, toggled)
     {
+        if (this.hasLineClass(lineNumber, className) === toggled)
+            return;
         var lineHandle = this._codeMirror.getLineHandle(lineNumber);
         if (!lineHandle)
             return;
@@ -1081,6 +1083,19 @@ WebInspector.CodeMirrorTextEditor.prototype = {
 
     /**
      * @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)