* 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
int opener_render_process_id,
                                     int opener_render_frame_id,
                                     const std::string& frame_name,
                                     const GURL& target_url,
                                     content::WebContents* new_contents) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto api_web_contents = CreateFrom(isolate(), new_contents, BROWSER_WINDOW);
  Emit("-web-contents-created", api_web_contents, target_url, frame_name);
}
void WebContents::AddNewContents(content::WebContents* source,
                                 content::WebContents* new_contents,
                                 WindowOpenDisposition disposition,
                                 const gfx::Rect& initial_rect,
                                 bool user_gesture,
                                 bool* was_blocked) {
  new ChildWebContentsTracker(new_contents);
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto api_web_contents = CreateFrom(isolate(), new_contents);
  if (Emit("-add-new-contents", api_web_contents, disposition, user_gesture,
      initial_rect.x(), initial_rect.y(), initial_rect.width(),
      initial_rect.height())) {
    api_web_contents->DestroyWebContents(true );
  }
}
content::WebContents* WebContents::OpenURLFromTab(