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
atom::api::WebContents::Type* out) {
    using Type = atom::api::WebContents::Type;
    std::string type;
    if (!ConvertFromV8(isolate, val, &type))
      return false;
    if (type == "webview") {
      *out = Type::WEB_VIEW;
    } else if (type == "backgroundPage") {
      *out = Type::BACKGROUND_PAGE;
    } else if (type == "offscreen") {
      *out = Type::OFF_SCREEN;
    } else {
      return false;
    }
    return true;
  }
};
}  
namespace atom {
namespace api {
namespace {
content::ServiceWorkerContext* GetServiceWorkerContext(