* 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
const content::WebContents* web_contents) {
  auto* context = web_contents->GetBrowserContext();
  auto* site_instance = web_contents->GetSiteInstance();
  if (!context || !site_instance)
    return nullptr;
  auto* storage_partition =
      content::BrowserContext::GetStoragePartition(context, site_instance);
  if (!storage_partition)
    return nullptr;
  return storage_partition->GetServiceWorkerContext();
}
void OnCapturePageDone(const base::Callback<void(const gfx::Image&)>& callback,
                       const SkBitmap& bitmap) {
  
  
  const_cast<SkBitmap&>(bitmap).setAlphaType(kPremul_SkAlphaType);
  callback.Run(gfx::Image::CreateFrom1xBitmap(bitmap));
}
}  
struct WebContents::FrameDispatchHelper {
  WebContents* api_web_contents;
  content::RenderFrameHost* rfh;
  bool Send(IPC::Message* msg) { return rfh->Send(msg); }
  void OnSetTemporaryZoomLevel(double level, IPC::Message* reply_msg) {
    api_web_contents->OnSetTemporaryZoomLevel(rfh, level, reply_msg);
  }
  void OnGetZoomLevel(IPC::Message* reply_msg) {
    api_web_contents->OnGetZoomLevel(rfh, reply_msg);
  }
  void OnRendererMessageSync(const std::string& channel,