DevTools: do not update ui on every warning update.

Source/devtools/front_end/ResponsiveDesignView.js

 // Copyright 2014 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.VBox}
  * @implements {WebInspector.OverridesSupport.PageResizer}
  * @param {!WebInspector.InspectedPagePlaceholder} inspectedPagePlaceholder
  */
@@ skipping 436 matching lines @@
 
         fieldsetElement.createChild("div", "responsive-design-icon responsive-design-icon-dpr").title = WebInspector.UIString("Device pixel ratio");
         fieldsetElement.appendChild(WebInspector.SettingsUI.createSettingInputField("", WebInspector.overridesSupport.settings.deviceScaleFactor, true, 2, "2em", WebInspector.OverridesSupport.inputValidator, true));
 
         // Touch and viewport
         fieldsetElement = this._toolbarElement.createChild("fieldset", "responsive-design-section");
         fieldsetElement.appendChild(WebInspector.SettingsUI.createSettingCheckbox(WebInspector.UIString("Touch"), WebInspector.overridesSupport.settings.emulateTouchEvents, true));
         fieldsetElement.appendChild(WebInspector.SettingsUI.createSettingCheckbox(WebInspector.UIString("Viewport"), WebInspector.overridesSupport.settings.emulateViewport, true));
 
         // Warning
-        this._warningMessage = this._responsiveDesignContainer.element.createChild("div", "responsive-design-warning");
+        this._warningMessage = this._responsiveDesignContainer.element.createChild("div", "responsive-design-warning hidden");
         WebInspector.overridesSupport.addEventListener(WebInspector.OverridesSupport.Events.OverridesWarningUpdated, this._overridesWarningUpdated, this);
     },
 
     _overridesWarningUpdated: function()
     {
         var message = WebInspector.overridesSupport.warningMessage();
+        if (this._warningMessage.textContent === message)
+            return;
         this._warningMessage.classList.toggle("hidden", !message);
         this._warningMessage.textContent = message;
         this._responsiveDesignModeChanged();
         this.onResize();
     },
 
     __proto__: WebInspector.VBox.prototype
 };