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
v8::Local<v8::FunctionTemplate> prototype) {
  prototype->SetClassName(mate::StringToV8(isolate, "WebContents"));
  mate::ObjectTemplateBuilder(isolate, prototype->PrototypeTemplate())
      .MakeDestroyable()
      .SetMethod("getId", &WebContents::GetID)
      .SetMethod("getProcessId", &WebContents::GetProcessID)
      .SetMethod("equal", &WebContents::Equal)
      .SetMethod("_loadURL", &WebContents::LoadURL)
      .SetMethod("downloadURL", &WebContents::DownloadURL)
      .SetMethod("_getURL", &WebContents::GetURL)
      .SetMethod("getTitle", &WebContents::GetTitle)
      .SetMethod("isLoading", &WebContents::IsLoading)
      .SetMethod("isLoadingMainFrame", &WebContents::IsLoadingMainFrame)
      .SetMethod("isWaitingForResponse", &WebContents::IsWaitingForResponse)
      .SetMethod("_stop", &WebContents::Stop)
      .SetMethod("_goBack", &WebContents::GoBack)
      .SetMethod("_goForward", &WebContents::GoForward)
      .SetMethod("_goToOffset", &WebContents::GoToOffset)
      .SetMethod("isCrashed", &WebContents::IsCrashed)
      .SetMethod("setUserAgent", &WebContents::SetUserAgent)
      .SetMethod("getUserAgent", &WebContents::GetUserAgent)
      .SetMethod("insertCSS", &WebContents::InsertCSS)
      .SetMethod("savePage", &WebContents::SavePage)
      .SetMethod("openDevTools", &WebContents::OpenDevTools)
      .SetMethod("closeDevTools", &WebContents::CloseDevTools)
      .SetMethod("isDevToolsOpened", &WebContents::IsDevToolsOpened)
      .SetMethod("isDevToolsFocused", &WebContents::IsDevToolsFocused)
      .SetMethod("enableDeviceEmulation",
                 &WebContents::EnableDeviceEmulation)