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
mate::Dictionary dict = mate::Dictionary::CreateEmpty(isolate);
  dict.Set("canUndo",
      !!(editFlags & blink::WebContextMenuData::CanUndo));
  dict.Set("canRedo",
      !!(editFlags & blink::WebContextMenuData::CanRedo));
  dict.Set("canCut",
      !!(editFlags & blink::WebContextMenuData::CanCut));
  dict.Set("canCopy",
      !!(editFlags & blink::WebContextMenuData::CanCopy));
  bool pasteFlag = false;
  if (editFlags & blink::WebContextMenuData::CanPaste) {
    std::vector<base::string16> types;
    bool ignore;
    ui::Clipboard::GetForCurrentThread()->ReadAvailableTypes(
        ui::CLIPBOARD_TYPE_COPY_PASTE, &types, &ignore);
    pasteFlag = !types.empty();
  }
  dict.Set("canPaste", pasteFlag);
  dict.Set("canDelete",
      !!(editFlags & blink::WebContextMenuData::CanDelete));
  dict.Set("canSelectAll",
      !!(editFlags & blink::WebContextMenuData::CanSelectAll));
  return mate::ConvertToV8(isolate, dict);
}
v8::Local<v8::Value> MediaFlagsToV8(v8::Isolate* isolate, int mediaFlags) {