### 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
content::BrowserContext* browser_context,
    const GURL& url,
    content::RenderFrameHost* rfh) const {
  std::string affinity = GetAffinityPreference(rfh);
  if (!affinity.empty()) {
    auto iter = site_per_affinities_.find(affinity);
    GURL dest_site = content::SiteInstance::GetSiteForURL(browser_context, url);
    if (iter != site_per_affinities_.end() &&
        IsSameWebSite(browser_context, iter->second, dest_site)) {
      return iter->second;
    }
  }
  return nullptr;
}
void AtomBrowserClient::ConsiderSiteInstanceForAffinity(
    content::RenderFrameHost* rfh,
    content::SiteInstance* site_instance) {
  std::string affinity = GetAffinityPreference(rfh);
  if (!affinity.empty()) {
    site_per_affinities_[affinity] = site_instance;
  }
}
bool AtomBrowserClient::IsRendererSubFrame(int process_id) const {
  return base::Contains(renderer_is_subframe_, process_id);
}
void AtomBrowserClient::RenderProcessWillLaunch(