* 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::NavigationHandle* navigation_handle) {
  if (!navigation_handle->IsInMainFrame() || !navigation_handle->HasCommitted())
    return;
  if (navigation_handle->IsErrorPage()) {
    content::HostZoomMap::SendErrorPageZoomLevelRefresh(web_contents());
    return;
  }
  SetZoomFactorOnNavigationIfNeeded(navigation_handle->GetURL());
}
void WebContentsZoomController::WebContentsDestroyed() {
  observers_.Clear();
  host_zoom_factor_.clear();
  embedder_zoom_controller_ = nullptr;
}
void WebContentsZoomController::RenderFrameHostChanged(
    content::RenderFrameHost* old_host,
    content::RenderFrameHost* new_host) {
  
  content::HostZoomMap* new_host_zoom_map =
      content::HostZoomMap::GetForWebContents(web_contents());
  if (new_host_zoom_map == host_zoom_map_)
    return;
  host_zoom_map_ = new_host_zoom_map;
  zoom_subscription_ = host_zoom_map_->AddZoomLevelChangedCallback(base::Bind(
      &WebContentsZoomController::OnZoomLevelChanged, base::Unretained(this)));
}
void WebContentsZoomController::SetZoomFactorOnNavigationIfNeeded(