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
int64_t GetID() const;
  int GetProcessID() const;
  Type GetType() const;
  bool Equal(const WebContents* web_contents) const;
  void LoadURL(const GURL& url, const mate::Dictionary& options);
  void DownloadURL(const GURL& url);
  GURL GetURL() const;
  base::string16 GetTitle() const;
  bool IsLoading() const;
  bool IsLoadingMainFrame() const;
  bool IsWaitingForResponse() const;
  void Stop();
  void ReloadIgnoringCache();
  void GoBack();
  void GoForward();
  void GoToOffset(int offset);
  bool IsCrashed() const;
  void SetUserAgent(const std::string& user_agent, mate::Arguments* args);
  std::string GetUserAgent();
  void InsertCSS(const std::string& css);
  bool SavePage(const base::FilePath& full_file_path,
                const content::SavePageType& save_type,
                const SavePageHandler::SavePageCallback& callback);
  void OpenDevTools(mate::Arguments* args);
  void CloseDevTools();
  bool IsDevToolsOpened();
  bool IsDevToolsFocused();
  void ToggleDevTools();
  void EnableDeviceEmulation(const blink::WebDeviceEmulationParams& params);
  void DisableDeviceEmulation();