* 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
}
  params.transition_type = ui::PAGE_TRANSITION_TYPED;
  params.should_clear_history_list = true;
  params.override_user_agent = content::NavigationController::UA_OVERRIDE_TRUE;
  web_contents()->GetController().LoadURLWithParams(params);
  
  
  
  auto* const view = web_contents()->GetRenderWidgetHostView();
  if (view) {
    auto* web_preferences = WebContentsPreferences::From(web_contents());
    std::string color_name;
    if (web_preferences->GetPreference(options::kBackgroundColor,
                                       &color_name)) {
      view->SetBackgroundColor(ParseHexColor(color_name));
    } else {
      view->SetBackgroundColor(SK_ColorTRANSPARENT);
    }
  }
}
void WebContents::DownloadURL(const GURL& url) {
  auto* browser_context = web_contents()->GetBrowserContext();
  auto* download_manager =
      content::BrowserContext::GetDownloadManager(browser_context);
  std::unique_ptr<download::DownloadUrlParameters> download_params(
      content::DownloadRequestUtils::CreateDownloadForWebContentsMainFrame(
          web_contents(), url, NO_TRAFFIC_ANNOTATION_YET));
  download_manager->DownloadUrl(std::move(download_params));
}
GURL WebContents::GetURL() const {