[DevTools] UI improvements for responsive design view.

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
  */
 WebInspector.ResponsiveDesignView = function(inspectedPagePlaceholder)
 {
     WebInspector.VBox.call(this);
+    this.setMinimumSize(150, 150);
     this.registerRequiredCSS("responsiveDesignView.css");
 
     this._responsiveDesignContainer = new WebInspector.VBox();
 
     this._createToolbar();
     this._warningMessage = this._responsiveDesignContainer.element.createChild("div", "responsive-design-warning hidden");
     WebInspector.overridesSupport.addEventListener(WebInspector.OverridesSupport.Events.OverridesWarningUpdated, this._overridesWarningUpdated, this);
 
     this._canvasContainer = new WebInspector.View();
     this._canvasContainer.element.classList.add("responsive-design");
@@ skipping 434 matching lines @@
         if (this._warningMessage.textContent === message)
             return;
         this._warningMessage.classList.toggle("hidden", !message);
         this._warningMessage.textContent = message;
         this._invalidateCache();
         this.onResize();
     },
 
     __proto__: WebInspector.VBox.prototype
 };