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/common/native_mate_converters/blink_converter.cc
v8::Isolate* isolate,
      const blink::WebContextMenuData::InputFieldType& in) {
  switch (in) {
    case blink::WebContextMenuData::InputFieldTypePlainText:
      return mate::StringToV8(isolate, "plainText");
    case blink::WebContextMenuData::InputFieldTypePassword:
      return mate::StringToV8(isolate, "password");
    case blink::WebContextMenuData::InputFieldTypeOther:
      return mate::StringToV8(isolate, "other");
    default:
      return mate::StringToV8(isolate, "none");
  }
}
v8::Local<v8::Value> EditFlagsToV8(v8::Isolate* isolate, int editFlags) {