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
.SetMethod("disableDeviceEmulation",
                 &WebContents::DisableDeviceEmulation)
      .SetMethod("toggleDevTools", &WebContents::ToggleDevTools)
      .SetMethod("inspectElement", &WebContents::InspectElement)
      .SetMethod("setAudioMuted", &WebContents::SetAudioMuted)
      .SetMethod("isAudioMuted", &WebContents::IsAudioMuted)
      .SetMethod("undo", &WebContents::Undo)
      .SetMethod("redo", &WebContents::Redo)
      .SetMethod("cut", &WebContents::Cut)
      .SetMethod("copy", &WebContents::Copy)
      .SetMethod("paste", &WebContents::Paste)
      .SetMethod("pasteAndMatchStyle", &WebContents::PasteAndMatchStyle)
      .SetMethod("delete", &WebContents::Delete)
      .SetMethod("selectAll", &WebContents::SelectAll)
      .SetMethod("unselect", &WebContents::Unselect)
      .SetMethod("replace", &WebContents::Replace)
      .SetMethod("replaceMisspelling", &WebContents::ReplaceMisspelling)
      .SetMethod("findInPage", &WebContents::FindInPage)
      .SetMethod("stopFindInPage", &WebContents::StopFindInPage)
      .SetMethod("focus", &WebContents::Focus)
      .SetMethod("isFocused", &WebContents::IsFocused)
      .SetMethod("tabTraverse", &WebContents::TabTraverse)
      .SetMethod("_send", &WebContents::SendIPCMessage)
      .SetMethod("sendInputEvent", &WebContents::SendInputEvent)
      .SetMethod("beginFrameSubscription",
                 &WebContents::BeginFrameSubscription)
      .SetMethod("endFrameSubscription", &WebContents::EndFrameSubscription)
      .SetMethod("startDrag", &WebContents::StartDrag)
      .SetMethod("setSize", &WebContents::SetSize)
      .SetMethod("isGuest", &WebContents::IsGuest)