### 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
DCHECK(!g_browser_client);
  g_browser_client = this;
}
AtomBrowserClient::~AtomBrowserClient() {
  DCHECK(g_browser_client);
  g_browser_client = nullptr;
}
content::WebContents* AtomBrowserClient::GetWebContentsFromProcessID(
    int process_id) {
  
  
  if (base::Contains(pending_processes_, process_id))
    return pending_processes_[process_id];
  
  
  return WebContentsPreferences::GetWebContentsFromProcessID(process_id);
}
bool AtomBrowserClient::ShouldForceNewSiteInstance(
    content::RenderFrameHost* current_rfh,
    content::RenderFrameHost* speculative_rfh,
    content::BrowserContext* browser_context,
    const GURL& url,
    bool has_response_started) const {
  if (url.SchemeIs(url::kJavaScriptScheme))
    
    return false;
  content::SiteInstance* current_instance = current_rfh->GetSiteInstance();
  content::SiteInstance* speculative_instance =