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
return false;
  bool shifted = false;
  ui::KeyboardCode keyCode = atom::KeyboardCodeFromStr(str, &shifted);
  out->windowsKeyCode = keyCode;
  if (shifted)
    out->modifiers |= blink::WebInputEvent::ShiftKey;
  out->setKeyIdentifierFromWindowsKeyCode();
  ui::DomCode domCode = ui::UsLayoutKeyboardCodeToDomCode(keyCode);
  out->domCode = static_cast<int>(domCode);
  ui::DomKey domKey;
  ui::KeyboardCode dummy_code;
  int flags = atom::WebEventModifiersToEventFlags(out->modifiers);
  if (ui::DomCodeToUsLayoutDomKey(domCode, flags, &domKey, &dummy_code))
    out->domKey = static_cast<int>(domKey);
  if ((out->type == blink::WebInputEvent::Char ||
       out->type == blink::WebInputEvent::RawKeyDown)) {
    
    
    size_t text_length_cap = blink::WebKeyboardEvent::textLengthCap;
    base::string16 text16 = base::UTF8ToUTF16(str);
    memset(out->text, 0, text_length_cap);
    memset(out->unmodifiedText, 0, text_length_cap);
    for (size_t i = 0; i < std::min(text_length_cap, text16.size()); ++i) {
      out->text[i] = text16[i];
      out->unmodifiedText[i] = text16[i];
    }
  }
  return true;
}
bool Converter<content::NativeWebKeyboardEvent>::FromV8(