[Win][WebKitTestRunner] Implement EventSenderProxy
WebKitTestRunner for Windows port has been resurrected in Bug 189257,
but EventSenderProxy is not implemented yet at all.
A lot of test cases are failing, for example
Created attachment 359998 [details]
Comment on attachment 359998 [details]
Clearing flags on attachment: 359998
Committed r240460: <https://trac.webkit.org/changeset/240460>
All reviewed patches have been landed. Closing bug.