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::RenderFrameHost* frame,
    const content::BluetoothChooser::EventHandler& event_handler) {
  std::unique_ptr<BluetoothChooser> bluetooth_chooser(
      new BluetoothChooser(this, event_handler));
  return std::move(bluetooth_chooser);
}
void WebContents::BeforeUnloadFired(const base::TimeTicks& proceed_time) {
  
  
}
void WebContents::RenderViewDeleted(content::RenderViewHost* render_view_host) {
  Emit("render-view-deleted", render_view_host->GetProcess()->GetID());
}
void WebContents::RenderProcessGone(base::TerminationStatus status) {
  Emit("crashed", status == base::TERMINATION_STATUS_PROCESS_WAS_KILLED);
}
void WebContents::PluginCrashed(const base::FilePath& plugin_path,
                                base::ProcessId plugin_pid) {
  content::WebPluginInfo info;
  auto plugin_service = content::PluginService::GetInstance();
  plugin_service->GetPluginInfoByPath(plugin_path, &info);
  Emit("plugin-crashed", info.name, info.version);
}
void WebContents::MediaStartedPlaying(const MediaPlayerId& id) {