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
const GURL& target_url,
    const std::string& frame_name,
    WindowOpenDisposition disposition,
    const std::vector<base::string16>& features,
    const scoped_refptr<content::ResourceRequestBodyImpl>& body) {
  if (type_ == BROWSER_WINDOW || type_ == OFF_SCREEN)
    Emit("-new-window", target_url, frame_name, disposition, features, body);
  else
    Emit("new-window", target_url, frame_name, disposition, features);
}
void WebContents::WebContentsCreated(content::WebContents* source_contents,
                                     int opener_render_frame_id,
                                     const std::string& frame_name,
                                     const GURL& target_url,
                                     content::WebContents* new_contents) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto api_web_contents = CreateFrom(isolate(), new_contents, BROWSER_WINDOW);
  Emit("-web-contents-created", api_web_contents, target_url, frame_name);
}
void WebContents::AddNewContents(content::WebContents* source,