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
Emit("media-started-playing");
}
void WebContents::MediaStoppedPlaying(const MediaPlayerId& id) {
  Emit("media-paused");
}
void WebContents::DidChangeThemeColor(SkColor theme_color) {
  Emit("did-change-theme-color", atom::ToRGBHex(theme_color));
}
void WebContents::DocumentLoadedInFrame(
    content::RenderFrameHost* render_frame_host) {
  if (!render_frame_host->GetParent())
    Emit("dom-ready");
}
void WebContents::DidFinishLoad(content::RenderFrameHost* render_frame_host,
                                const GURL& validated_url) {
  bool is_main_frame = !render_frame_host->GetParent();
  Emit("did-frame-finish-load", is_main_frame);
  if (is_main_frame)
    Emit("did-finish-load");
}
void WebContents::DidFailLoad(content::RenderFrameHost* render_frame_host,
                              const GURL& url,
                              int error_code,
                              const base::string16& error_description,
                              bool was_ignored_by_handler) {
  bool is_main_frame = !render_frame_host->GetParent();
  Emit("did-fail-load", error_code, error_description, url, is_main_frame);
}
void WebContents::DidStartLoading() {