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
void OnEnterFullscreenModeForTab(content::WebContents* source,
                                   const GURL& origin,
                                   bool allowed);
  
  void OnCreateWindow(
      const GURL& target_url,
      const std::string& frame_name,
      WindowOpenDisposition disposition,
      const std::vector<base::string16>& features,
      const scoped_refptr<content::ResourceRequestBodyImpl>& body);
  
  v8::Local<v8::Value> GetWebPreferences(v8::Isolate* isolate);
  
  v8::Local<v8::Value> GetOwnerBrowserWindow();
  
  int32_t ID() const;
  v8::Local<v8::Value> Session(v8::Isolate* isolate);
  content::WebContents* HostWebContents();
  v8::Local<v8::Value> DevToolsWebContents(v8::Isolate* isolate);
  v8::Local<v8::Value> Debugger(v8::Isolate* isolate);
 protected:
  WebContents(v8::Isolate* isolate,
              content::WebContents* web_contents,
              Type type);
  WebContents(v8::Isolate* isolate, const mate::Dictionary& options);
  ~WebContents();
  void InitWithSessionAndOptions(v8::Isolate* isolate,
                                 content::WebContents *web_contents,
                                 mate::Handle<class Session> session,