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
CommonWebContentsDelegate::ExitFullscreenModeForTab(source);
  Emit("leave-html-full-screen");
}
void WebContents::RendererUnresponsive(content::WebContents* source) {
  Emit("unresponsive");
  if ((type_ == BROWSER_WINDOW || type_ == OFF_SCREEN) && owner_window())
    owner_window()->RendererUnresponsive(source);
}
void WebContents::RendererResponsive(content::WebContents* source) {
  Emit("responsive");
  if ((type_ == BROWSER_WINDOW || type_ == OFF_SCREEN) && owner_window())
    owner_window()->RendererResponsive(source);
}
bool WebContents::HandleContextMenu(const content::ContextMenuParams& params) {
  if (params.custom_context.is_pepper_menu) {
    Emit("pepper-context-menu", std::make_pair(params, web_contents()));
    web_contents()->NotifyContextMenuClosed(params.custom_context);
  } else {
    Emit("context-menu", std::make_pair(params, web_contents()));
  }
  return true;
}
bool WebContents::OnGoToEntryOffset(int offset) {