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
atom::AtomBrowserClient::SuppressRendererProcessRestartForOnce();
  web_contents()->GetController().GoForward();
}
void WebContents::GoToOffset(int offset) {
  atom::AtomBrowserClient::SuppressRendererProcessRestartForOnce();
  web_contents()->GetController().GoToOffset(offset);
}
bool WebContents::IsCrashed() const {
  return web_contents()->IsCrashed();
}
void WebContents::SetUserAgent(const std::string& user_agent,
                               mate::Arguments* args) {
  web_contents()->SetUserAgentOverride(user_agent);
}
std::string WebContents::GetUserAgent() {
  return web_contents()->GetUserAgentOverride();
}
void WebContents::InsertCSS(const std::string& css) {
  web_contents()->InsertCSS(css);
}
bool WebContents::SavePage(const base::FilePath& full_file_path,
                           const content::SavePageType& save_type,
                           const SavePageHandler::SavePageCallback& callback) {
  auto handler = new SavePageHandler(web_contents(), callback);
  return handler->Handle(full_file_path, save_type);
}
void WebContents::OpenDevTools(mate::Arguments* args) {