### 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.h
content::NavigationUIData* navigation_data,
      bool is_main_frame,
      ui::PageTransition page_transition,
      bool has_user_gesture,
      network::mojom::URLLoaderFactoryPtr* out_factory) override;
  
  void RenderProcessHostDestroyed(content::RenderProcessHost* host) override;
  void RenderProcessReady(content::RenderProcessHost* host) override;
  void RenderProcessExited(
      content::RenderProcessHost* host,
      const content::ChildProcessTerminationInfo& info) override;
 private:
  struct ProcessPreferences {
    bool sandbox = false;
    bool native_window_open = false;
    bool disable_popups = false;
    bool web_security = true;
  };
  bool ShouldForceNewSiteInstance(content::RenderFrameHost* current_rfh,
                                  content::RenderFrameHost* speculative_rfh,
                                  content::BrowserContext* browser_context,
                                  const GURL& dest_url,
                                  bool has_request_started) const;
  bool NavigationWasRedirectedCrossSite(
      content::BrowserContext* browser_context,
      content::SiteInstance* current_instance,
      content::SiteInstance* speculative_instance,
      const GURL& dest_url,
      bool has_request_started) const;
  void AddProcessPreferences(int process_id, ProcessPreferences prefs);