* Electron version: 1.7.5

### Expected behavior

When a theme color is no longer present the webContents did-change-theme-color event should return `null`.

### Actual behavior

The event returns `"#000000"`.

### How to reproduce

1. Log the returned color of the webContents did-change-theme-color event.
2. Go to https://electron.atom.io which has a theme color.
3. Go to https://atom.io which does not have a theme color.




atom/browser/api/atom_api_web_contents.cc
RemoveFromWeakMap();
  
  
  MarkDestroyed();
  Emit("destroyed");
  
  base::ThreadTaskRunnerHandle::Get()->PostTask(
      FROM_HERE, GetDestroyClosure());
}
void WebContents::NavigationEntryCommitted(
    const content::LoadCommittedDetails& details) {
  Emit("navigation-entry-commited", details.entry->GetURL(),
       details.is_in_page, details.did_replace_entry);
}
int64_t WebContents::GetID() const {
  int64_t process_id = web_contents()->GetRenderProcessHost()->GetID();
  int64_t routing_id = web_contents()->GetRenderViewHost()->GetRoutingID();
  int64_t rv = (process_id << 32) + routing_id;
  return rv;
}
int WebContents::GetProcessID() const {
  return web_contents()->GetRenderProcessHost()->GetID();
}
base::ProcessId WebContents::GetOSProcessID() const {
  auto process_handle = web_contents()->GetRenderProcessHost()->GetHandle();
  return base::GetProcId(process_handle);
}
WebContents::Type WebContents::GetType() const {