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
const mate::Dictionary& options);
  
  bool AddMessageToConsole(content::WebContents* source,
                           int32_t level,
                           const base::string16& message,
                           int32_t line_no,
                           const base::string16& source_id) override;
  void WebContentsCreated(content::WebContents* source_contents,
                          int opener_render_frame_id,
                          const std::string& frame_name,
                          const GURL& target_url,
                          content::WebContents* new_contents) override;
  void AddNewContents(content::WebContents* source,
                      content::WebContents* new_contents,
                      WindowOpenDisposition disposition,