* 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
WebContentsZoomController* controller) {
  embedder_zoom_controller_ = controller;
}
void WebContentsZoomController::SetZoomLevel(double level) {
  if (!web_contents()->GetRenderViewHost()->IsRenderViewLive() ||
      content::ZoomValuesEqual(GetZoomLevel(), level))
    return;
  int render_process_id = web_contents()->GetRenderProcessHost()->GetID();
  int render_view_id = web_contents()->GetRenderViewHost()->GetRoutingID();
  if (host_zoom_map_->UsesTemporaryZoomLevel(render_process_id,
                                             render_view_id)) {
    host_zoom_map_->ClearTemporaryZoomLevel(render_process_id, render_view_id);
  }
  auto new_zoom_factor = content::ZoomLevelToZoomFactor(level);
  content::NavigationEntry* entry =
      web_contents()->GetController().GetLastCommittedEntry();
  if (entry) {
    std::string host = net::GetHostOrSpecFromURL(entry->GetURL());
    
    if (!content::ZoomValuesEqual(GetDefaultZoomFactor(), new_zoom_factor))
      host_zoom_factor_[host] = new_zoom_factor;
    content::HostZoomMap::SetZoomLevel(web_contents(), level);
    
    for (Observer& observer : observers_)
      observer.OnZoomLevelChanged(web_contents(), level, false);
  }
}
double WebContentsZoomController::GetZoomLevel() {