* Electron version: 1.6.1
* Operating system: MacOS 10.12

Create a webview, set the zoom level, then create a second webview for content at the same domain as the 1st webview.

### Expected behavior

Ideally the second webview would assume the same zoom as the existing webview. At minumum, it'd be good if the first webview maintained its current zoom.

### Actual behavior

The zoom level is reset on both webviews.

### How to reproduce

https://github.com/pfrazee/electron-bug-webview-zoom Instructions on the readme
atom/browser/web_contents_zoom_controller.h
: public content::WebContentsObserver,
      public content::WebContentsUserData<WebContentsZoomController> {
 public:
  class Observer {
   public:
    virtual void OnZoomLevelChanged(content::WebContents* web_contents,
                                    double level,
                                    bool is_temporary) {}
   protected:
    virtual ~Observer() {}
  };
  explicit WebContentsZoomController(content::WebContents* web_contents);
  ~WebContentsZoomController() override;
  void AddObserver(Observer* observer);
  void RemoveObserver(Observer* observer);
  void SetEmbedderZoomController(WebContentsZoomController* controller);
  
  void SetZoomLevel(double level);
  double GetZoomLevel();
  void SetDefaultZoomFactor(double factor);
  double GetDefaultZoomFactor();