* 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
if (managed_web_contents())
    managed_web_contents()->SetDevToolsWebContents(devtools->web_contents());
}
v8::Local<v8::Value> WebContents::GetNativeView() const {
  gfx::NativeView ptr = web_contents()->GetNativeView();
  auto buffer = node::Buffer::Copy(isolate(), reinterpret_cast<char*>(&ptr),
                                   sizeof(gfx::NativeView));
  if (buffer.IsEmpty())
    return v8::Null(isolate());
  else
    return buffer.ToLocalChecked();
}
v8::Local<v8::Value> WebContents::DevToolsWebContents(v8::Isolate* isolate) {
  if (devtools_web_contents_.IsEmpty())
    return v8::Null(isolate);
  else
    return v8::Local<v8::Value>::New(isolate, devtools_web_contents_);
}
v8::Local<v8::Value> WebContents::Debugger(v8::Isolate* isolate) {
  if (debugger_.IsEmpty()) {
    auto handle = atom::api::Debugger::Create(isolate, web_contents());
    debugger_.Reset(isolate, handle.ToV8());
  }
  return v8::Local<v8::Value>::New(isolate, debugger_);
}
void WebContents::GrantOriginAccess(const GURL& url) {