* 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
#include "atom/browser/web_contents_zoom_controller.h"
#include "content/public/browser/navigation_details.h"
#include "content/public/browser/navigation_entry.h"
#include "content/public/browser/navigation_handle.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/web_contents.h"
#include "content/public/common/page_type.h"
#include "content/public/common/page_zoom.h"
#include "net/base/url_util.h"
DEFINE_WEB_CONTENTS_USER_DATA_KEY(atom::WebContentsZoomController);
namespace atom {
WebContentsZoomController::WebContentsZoomController(