* 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
content::RenderFrameHost* frame,
    const content::BluetoothChooser::EventHandler& event_handler) {
  return std::make_unique<BluetoothChooser>(this, event_handler);
}
content::JavaScriptDialogManager* WebContents::GetJavaScriptDialogManager(
    content::WebContents* source) {
  if (!dialog_manager_)
    dialog_manager_.reset(new AtomJavaScriptDialogManager(this));
  return dialog_manager_.get();
}
void WebContents::OnAudioStateChanged(content::WebContents* web_contents,
                                      bool audible) {
  Emit("-audio-state-changed", audible);
}
void WebContents::BeforeUnloadFired(const base::TimeTicks& proceed_time) {
  
  
}
void WebContents::RenderViewCreated(content::RenderViewHost* render_view_host) {
  auto* const impl = content::RenderWidgetHostImpl::FromID(
      render_view_host->GetProcess()->GetID(),
      render_view_host->GetRoutingID());
  if (impl)
    impl->disable_hidden_ = !background_throttling_;
}
void WebContents::RenderViewDeleted(content::RenderViewHost* render_view_host) {
  Emit("render-view-deleted", render_view_host->GetProcess()->GetID(),
       base::GetProcId(render_view_host->GetProcess()->GetHandle()));
}
void WebContents::RenderProcessGone(base::TerminationStatus status) {