* 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.cc
content::WebContents* web_contents)
    : content::WebContentsObserver(web_contents),
      old_process_id_(-1),
      old_view_id_(-1),
      embedder_zoom_controller_(nullptr) {
  default_zoom_factor_ = content::kEpsilon;
  host_zoom_map_ = content::HostZoomMap::GetForWebContents(web_contents);
  zoom_subscription_ = host_zoom_map_->AddZoomLevelChangedCallback(base::Bind(
      &WebContentsZoomController::OnZoomLevelChanged, base::Unretained(this)));
}
WebContentsZoomController::~WebContentsZoomController() {}
void WebContentsZoomController::AddObserver(
    WebContentsZoomController::Observer* observer) {
  observers_.AddObserver(observer);
}
void WebContentsZoomController::RemoveObserver(
    WebContentsZoomController::Observer* observer) {
  observers_.RemoveObserver(observer);
}
void WebContentsZoomController::SetEmbedderZoomController(