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.h
bool final_update) override;
  bool CheckMediaAccessPermission(
      content::WebContents* web_contents,
      const GURL& security_origin,
      content::MediaStreamType type) override;
  void RequestMediaAccessPermission(
      content::WebContents* web_contents,
      const content::MediaStreamRequest& request,
      const content::MediaResponseCallback& callback) override;
  void RequestToLockMouse(
      content::WebContents* web_contents,
      bool user_gesture,
      bool last_unlocked_by_target) override;
  std::unique_ptr<content::BluetoothChooser> RunBluetoothChooser(
      content::RenderFrameHost* frame,
      const content::BluetoothChooser::EventHandler& handler) override;
  
  void BeforeUnloadFired(const base::TimeTicks& proceed_time) override;
  void RenderViewDeleted(content::RenderViewHost*) override;
  void RenderProcessGone(base::TerminationStatus status) override;
  void DocumentLoadedInFrame(
      content::RenderFrameHost* render_frame_host) override;
  void DidFinishLoad(content::RenderFrameHost* render_frame_host,
                     const GURL& validated_url) override;
  void DidFailLoad(content::RenderFrameHost* render_frame_host,
                   const GURL& validated_url,
                   int error_code,
                   const base::string16& error_description,
                   bool was_ignored_by_handler) override;
  void DidStartLoading() override;