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
web_contents()->StopFinding(action);
}
void WebContents::ShowDefinitionForSelection() {
#if defined(OS_MACOSX)
  const auto view = web_contents()->GetRenderWidgetHostView();
  if (view)
    view->ShowDefinitionForSelection();
#endif
}
void WebContents::CopyImageAt(int x, int y) {
  const auto host = web_contents()->GetMainFrame();
  if (host)
    host->CopyImageAt(x, y);
}
void WebContents::Focus() {
  web_contents()->Focus();
}
#if !defined(OS_MACOSX)
bool WebContents::IsFocused() const {
  auto view = web_contents()->GetRenderWidgetHostView();
  if (!view) return false;
  if (GetType() != BACKGROUND_PAGE) {
    auto window = web_contents()->GetNativeView()->GetToplevelWindow();
    if (window && !window->IsVisible())
      return false;
  }
  return view->HasFocus();
}
#endif
void WebContents::TabTraverse(bool reverse) {