* 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
int opener_render_process_id,
                                     int opener_render_frame_id,
                                     const std::string& frame_name,
                                     const GURL& target_url,
                                     content::WebContents* new_contents) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto api_web_contents = CreateFrom(isolate(), new_contents, BROWSER_WINDOW);
  Emit("-web-contents-created", api_web_contents, target_url, frame_name);
}
void WebContents::AddNewContents(content::WebContents* source,
                                 content::WebContents* new_contents,
                                 WindowOpenDisposition disposition,
                                 const gfx::Rect& initial_rect,
                                 bool user_gesture,
                                 bool* was_blocked) {
  new ChildWebContentsTracker(new_contents);
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto api_web_contents = CreateFrom(isolate(), new_contents);
  if (Emit("-add-new-contents", api_web_contents, disposition, user_gesture,
           initial_rect.x(), initial_rect.y(), initial_rect.width(),
           initial_rect.height())) {
    api_web_contents->DestroyWebContents(true );
  }
}
content::WebContents* WebContents::OpenURLFromTab(