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,
                                            mate::Handle<api::Session> session,
                                            const mate::Dictionary& options) {
  Observe(web_contents);
  InitWithWebContents(web_contents, session->browser_context());
  managed_web_contents()->GetView()->SetDelegate(this);
  
  new WebContentsPreferences(web_contents, options);
  
  WebContentsPermissionHelper::CreateForWebContents(web_contents);
  
  AtomSecurityStateModelClient::CreateForWebContents(web_contents);
  web_contents->SetUserAgentOverride(GetBrowserContext()->GetUserAgent());
  if (IsGuest()) {
    guest_delegate_->Initialize(this);
    NativeWindow* owner_window = nullptr;
    if (options.Get("embedder", &embedder_) && embedder_) {
      
      auto relay =
          NativeWindowRelay::FromWebContents(embedder_->web_contents());
      if (relay)
        owner_window = relay->window.get();
    }
    if (owner_window)
      SetOwnerWindow(owner_window);
  }
  Init(isolate);
  AttachAsUserData(web_contents);
}
WebContents::~WebContents() {