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
int bits = 0;
  for (const T& item : vec)
    bits |= item;
  return bits;
}
}  
namespace mate {
template<>
struct Converter<base::char16> {
  static bool FromV8(v8::Isolate* isolate, v8::Handle<v8::Value> val,
                     base::char16* out) {
    base::string16 code = base::UTF8ToUTF16(V8ToString(val));
    if (code.length() != 1)
      return false;
    *out = code[0];
    return true;
  }
};
template<>
struct Converter<blink::WebInputEvent::Type> {
  static bool FromV8(v8::Isolate* isolate, v8::Handle<v8::Value> val,