Use the typed enum CefEventFlags for modifiers in keyboard events.

Cef says for the `modifier` field:
> See cef_event_flags_t for values.

So I think it is safe to cast the value to the existing `CefEventFlags` enum type.

With this PR, I am now able to use the following logic in my app:

```C#
        bool IKeyboardHandler.OnPreKeyEvent(IWebBrowser browser, KeyType type, int windowsKeyCode, int nativeKeyCode, CefEventFlags modifiers, bool isSystemKey, bool isKeyboardShortcut)
        {
                Keys keys = (Keys)windowsKeyCode;
                if ((modifiers & CefEventFlags.ControlDown) != 0)
                    keys |= Keys.Control;
                else if ((modifiers & CefEventFlags.AltDown) != 0)
                    keys |= Keys.Alt;
                else if ((modifiers & CefEventFlags.ShiftDown) != 0)
                    keys |= Keys.Shift;
```