- Electron version: 1.2.8
- Operating system: Windows

Hi,

I've been trying to get a context menu containing some [predefined web contents roles](https://github.com/electron/electron/blob/master/lib/browser/api/menu-item-roles.js#L17) working for an application where we have multiple windows simultaneously. I've noticed that when a context menu is opened for a window that was created later than others, the role commands don't seem to execute when clicked. However, if don't use a role and instead define a click handler that calls the command on the web contents manually, it behaves normally.

I've been doing some digging and it seems that the reason is because, for a role, [`electron.webContents.getFocusedWebContents`](https://github.com/electron/electron/blob/master/lib/browser/api/web-contents.js#L257) is used to determine which web contents to call the command on (see [menu.js](https://github.com/electron/electron/blob/master/lib/browser/api/menu.js#L121)).

This function iterates through all existing web contents, then returns the first in the collection that is in a `webview`, otherwise the first that returns `true` from `contents.isFocused()`. However, it seems to be entirely possible that there are web contents other than the one that the context menu was raised for, who think they are focused (or perhaps are within a `webview`), and who were created earlier and therefore appear earlier in the collection.

This means there are situations when the 'wrong' web contents is returned from `getFocusedWebContents`, and therefore the role command is called on that 'wrong' web contents, which leads to the unexpected behaviour described above.

Is this a bug? And if so, is there a fix and/or workaround for this? I'm happy to contribute a PR if needed.

atom/browser/api/atom_api_web_contents.cc
const content::WebContents* web_contents) {
  auto context = web_contents->GetBrowserContext();
  auto site_instance = web_contents->GetSiteInstance();
  if (!context || !site_instance)
    return nullptr;
  auto storage_partition =
      content::BrowserContext::GetStoragePartition(context, site_instance);
  if (!storage_partition)
    return nullptr;
  return storage_partition->GetServiceWorkerContext();
}
void OnCapturePageDone(base::Callback<void(const gfx::Image&)> callback,
                       const SkBitmap& bitmap,
                       content::ReadbackResponse response) {
  callback.Run(gfx::Image::CreateFrom1xBitmap(bitmap));
}
}  
WebContents::WebContents(v8::Isolate* isolate,
                         content::WebContents* web_contents)
    : content::WebContentsObserver(web_contents),
      embedder_(nullptr),
      type_(REMOTE),
      request_id_(0),
      background_throttling_(true) {
  web_contents->SetUserAgentOverride(GetBrowserContext()->GetUserAgent());
  Init(isolate);
  AttachAsUserData(web_contents);
}
WebContents::WebContents(v8::Isolate* isolate,