* Electron Version: v3.0.0-nightly.20180818
* Operating System (Platform and Version): Windows 10 1803
* Last known working Electron version: v3.0.0-beta.5

**Expected Behavior**
webview will not be freezed

**Actual behavior**
webview freezed occasionlly on navigation

**To Reproduce**
1. clone https://github.com/KochiyaOcean/electron-bugs

2. run `electron .` using electron v3.0.0-nightly.20180818

3. run scripts below in devtools
```javascript
require('electron').remote.webContents.getAllWebContents()
```
![image](https://user-images.githubusercontent.com/8194131/44320386-bf77ce80-a47c-11e8-9371-4e89cec444db.png)
there are 3 webcontents (BrowserWindow, WebView, DevTool)

4. run scripts below in devtools
```javascript
document.querySelector('webview').addEventListener('did-finish-load', () => document.querySelector('webview').loadURL('http://dmm.com'))
document.querySelector('webview').loadURL('http://dmm.com')
```

5. webview will freeze occasionally, and call any method of webview will throw an error
![image](https://user-images.githubusercontent.com/8194131/44320462-0b2a7800-a47d-11e8-9b21-3033df0372c4.png)

6. run `require('electron').remote.webContents.getAllWebContents()` again, and there are only 2 webcontents left (webcontents of webview is removed)
![image](https://user-images.githubusercontent.com/8194131/44320488-3c0aad00-a47d-11e8-99db-634428b1198c.png)

**Additional Information**
This bug occurs easier on complex websites, such as https://amazon.com , https://ebay.com , etc.
atom/browser/api/atom_api_web_contents.cc
#include "content/public/browser/storage_partition.h"
#include "content/public/browser/web_contents.h"
#include "content/public/common/context_menu_params.h"
#include "native_mate/converter.h"
#include "native_mate/dictionary.h"
#include "native_mate/object_template_builder.h"
#include "net/url_request/url_request_context.h"
#include "third_party/WebKit/public/platform/WebInputEvent.h"
#include "third_party/WebKit/public/web/WebFindOptions.h"
#include "ui/display/screen.h"
#include "ui/events/base_event_utils.h"
#if !defined(OS_MACOSX)
#include "ui/aura/window.h"
#endif
#if defined(OS_LINUX) || defined(OS_WIN)
#include "content/public/common/renderer_preferences.h"
#include "ui/gfx/font_render_params.h"
#endif
#include "atom/common/node_includes.h"
namespace {
struct PrintSettings {
  bool silent;
  bool print_background;
  base::string16 device_name;
};
}  
namespace mate {
template <>
struct Converter<PrintSettings> {
  static bool FromV8(v8::Isolate* isolate,