### Issue Details

* **Electron Version:**
  * 3.x, 4.x, 5.x, 6.x
* **Operating System:**
  * All
* **Last Known Working Electron version:**:
  * 2.x

### Expected Behavior
When navigations occur, a new process should be created for the navigation.

### Actual Behavior
When a navigation occurs _during_ a preloads scripts execution the same process is re-used as Chromium does not think the navigation has been committed yet.  This causes a memory leak and all native node modules to break.

### To Reproduce
Fiddle: https://gist.github.com/MarshallOfSound/c89aa0037c3da114d44aa3a2d1a0fc24

### Screenshots
![image](https://user-images.githubusercontent.com/6634592/55097842-eb45dd00-5079-11e9-99bf-d49c0d997bca.png)

As you can see here there are three navigation loads, the second and the third one actually have the same process ID (and are running in the same process)
shell/browser/atom_browser_client.cc
const GURL& url,
    content::WebContents::Getter web_contents_getter,
    int child_id,
    content::NavigationUIData* navigation_data,
    bool is_main_frame,
    ui::PageTransition page_transition,
    bool has_user_gesture,
    network::mojom::URLLoaderFactoryPtr* out_factory) {
  base::PostTaskWithTraits(
      FROM_HERE, {BrowserThread::UI},
      base::BindOnce(&HandleExternalProtocolInUI, url, web_contents_getter,
                     has_user_gesture));
  return true;
}
std::vector<std::unique_ptr<content::NavigationThrottle>>
AtomBrowserClient::CreateThrottlesForNavigation(
    content::NavigationHandle* handle) {
  std::vector<std::unique_ptr<content::NavigationThrottle>> throttles;
  throttles.push_back(std::make_unique<AtomNavigationThrottle>(handle));
  return throttles;
}
content::MediaObserver* AtomBrowserClient::GetMediaObserver() {