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
content::WebContents* new_contents,
                                 WindowOpenDisposition disposition,
                                 const gfx::Rect& initial_rect,
                                 bool user_gesture,
                                 bool* was_blocked) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto api_web_contents = CreateFrom(isolate(), new_contents);
  Emit("-add-new-contents", api_web_contents, disposition, user_gesture,
      initial_rect.x(), initial_rect.y(), initial_rect.width(),
      initial_rect.height());
}
content::WebContents* WebContents::OpenURLFromTab(
    content::WebContents* source,
    const content::OpenURLParams& params) {
  if (params.disposition != CURRENT_TAB) {
    if (type_ == BROWSER_WINDOW || type_ == OFF_SCREEN)
      Emit("-new-window", params.url, "", params.disposition);
    else
      Emit("new-window", params.url, "", params.disposition);
    return nullptr;
  }
  
  if (Emit("will-navigate", params.url))
    return nullptr;
  return CommonWebContentsDelegate::OpenURLFromTab(source, params);
}
void WebContents::BeforeUnloadFired(content::WebContents* tab,