* 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
return content::HostZoomMap::GetZoomLevel(web_contents());
}
void WebContentsZoomController::SetDefaultZoomFactor(double factor) {
  default_zoom_factor_ = factor;
}
double WebContentsZoomController::GetDefaultZoomFactor() {
  return default_zoom_factor_;
}
void WebContentsZoomController::SetTemporaryZoomLevel(double level) {
  old_process_id_ = web_contents()->GetRenderProcessHost()->GetID();
  old_view_id_ = web_contents()->GetRenderViewHost()->GetRoutingID();
  host_zoom_map_->SetTemporaryZoomLevel(old_process_id_, old_view_id_, level);
  
  for (Observer& observer : observers_)
    observer.OnZoomLevelChanged(web_contents(), level, true);
}
bool WebContentsZoomController::UsesTemporaryZoomLevel() {
  int render_process_id = web_contents()->GetRenderProcessHost()->GetID();
  int render_view_id = web_contents()->GetRenderViewHost()->GetRoutingID();
  return host_zoom_map_->UsesTemporaryZoomLevel(render_process_id,
                                                render_view_id);
}
void WebContentsZoomController::DidFinishNavigation(