Embedding arbitrary web content is problematic when it comes to keyboard shortcuts because:
- Web content can steal app shortcuts (see e.g. brave/browser-laptop#4407)
- Blocked web content (e.g. a focused `<webview>` performing expensive computation) will also prevent app shortcuts from firing immediately

To fix this, we need a way to _always_ handle certain shortcuts in the main process. I think the most flexible solution would be something like this:

``` js
app.addProtectedShortcut('CmdOrCtrl+Tab', (event) => {
  event.preventDefault();
});
```

We could then implement:

``` cpp
bool CommonWebContentsDelegate::PreHandleKeyboardEvent(
    content::WebContents* source,
    const content::NativeWebKeyboardEvent& event,
    bool* is_keyboard_shortcut) {
  return /* event is protected shortcut and the handler called preventDefault */;
}
```

@zcbenz Thoughts? I'm happy to work on this if we come up with a reasonable approach.

atom/browser/api/atom_api_web_contents.cc
content::WebContents* web_contents,
    const GURL& security_origin,
    content::MediaStreamType type) {
  return true;
}
void WebContents::RequestMediaAccessPermission(
    content::WebContents* web_contents,
    const content::MediaStreamRequest& request,
    const content::MediaResponseCallback& callback) {
  auto permission_helper =
      WebContentsPermissionHelper::FromWebContents(web_contents);
  permission_helper->RequestMediaAccessPermission(request, callback);
}
void WebContents::RequestToLockMouse(
    content::WebContents* web_contents,
    bool user_gesture,
    bool last_unlocked_by_target) {
  auto permission_helper =
      WebContentsPermissionHelper::FromWebContents(web_contents);
  permission_helper->RequestPointerLockPermission(user_gesture);
}
std::unique_ptr<content::BluetoothChooser> WebContents::RunBluetoothChooser(