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
const mate::Dictionary& options)
    : embedder_(nullptr),
      type_(BROWSER_WINDOW),
      request_id_(0),
      background_throttling_(true),
      enable_devtools_(true) {
  
  options.Get("backgroundThrottling", &background_throttling_);
  
  
  
  
  
  bool b = false;
  if (options.Get("isGuest", &b) && b)
    type_ = WEB_VIEW;
  else if (options.Get("isBackgroundPage", &b) && b)
    type_ = BACKGROUND_PAGE;
  else if (options.Get("offscreen", &b) && b)
    type_ = OFF_SCREEN;
  
  options.Get("devTools", &enable_devtools_);
  
  std::string partition;
  mate::Handle<api::Session> session;
  if (options.Get("session", &session)) {
  } else if (options.Get("partition", &partition)) {
    session = Session::FromPartition(isolate, partition);
  } else {