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
return static_cast<AtomBrowserContext*>(web_contents()->GetBrowserContext());
}
void WebContents::OnRendererMessage(const base::string16& channel,
                                    const base::ListValue& args) {
  
  Emit(base::UTF16ToUTF8(channel), args);
}
void WebContents::OnRendererMessageSync(const base::string16& channel,
                                        const base::ListValue& args,
                                        IPC::Message* message) {
  
  EmitWithSender(base::UTF16ToUTF8(channel), web_contents(), message, args);
}
mate::Handle<WebContents> WebContents::CreateFrom(
    v8::Isolate* isolate, content::WebContents* web_contents) {
  
  auto existing = TrackableObject::FromWrappedClass(isolate, web_contents);
  if (existing)
    return mate::CreateHandle(isolate, static_cast<WebContents*>(existing));
  
  return mate::CreateHandle(isolate, new WebContents(isolate, web_contents,
        REMOTE));
}
mate::Handle<WebContents> WebContents::CreateFrom(