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.h
void DidStopLoading() override;
  void DidGetResourceResponseStart(
      const content::ResourceRequestDetails& details) override;
  void DidGetRedirectForResourceRequest(
      content::RenderFrameHost* render_frame_host,
      const content::ResourceRedirectDetails& details) override;
  void DidFinishNavigation(
      content::NavigationHandle* navigation_handle) override;
  bool OnMessageReceived(const IPC::Message& message) override;
  void WebContentsDestroyed() override;
  void NavigationEntryCommitted(
      const content::LoadCommittedDetails& load_details) override;
  void TitleWasSet(content::NavigationEntry* entry, bool explicit_set) override;
  void DidUpdateFaviconURL(
      const std::vector<content::FaviconURL>& urls) override;
  void PluginCrashed(const base::FilePath& plugin_path,
                     base::ProcessId plugin_pid) override;
  void MediaStartedPlaying(const MediaPlayerId& id) override;
  void MediaStoppedPlaying(const MediaPlayerId& id) override;
  void DidChangeThemeColor(SkColor theme_color) override;
  
  void DevToolsReloadPage() override;
  
  void DevToolsFocused() override;
  void DevToolsOpened() override;
  void DevToolsClosed() override;
 private:
  AtomBrowserContext* GetBrowserContext() const;
  uint32_t GetNextRequestId() {
    return ++request_id_;