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 Focus();
  bool IsFocused() const;
  void TabTraverse(bool reverse);
  
  bool SendIPCMessage(bool all_frames,
                      const base::string16& channel,
                      const base::ListValue& args);
  
  void SendInputEvent(v8::Isolate* isolate, v8::Local<v8::Value> input_event);
  
  void BeginFrameSubscription(mate::Arguments* args);
  void EndFrameSubscription();
  
  void StartDrag(const mate::Dictionary& item, mate::Arguments* args);
  
  
  void CapturePage(mate::Arguments* args);
  
  void SetSize(const SetSizeParams& params);
  bool IsGuest() const;
  
  bool IsOffScreen() const;
  void OnPaint(const gfx::Rect& dirty_rect, const SkBitmap& bitmap);
  void StartPainting();
  void StopPainting();
  bool IsPainting() const;
  void SetFrameRate(int frame_rate);
  int GetFrameRate() const;
  void Invalidate();