Simplify context menu handling on Windows

Simplify context menu handling on Windows
https://bugs.webkit.org/show_bug.cgi?id=150423

Reviewed by Tim Horton.

Instead of converting a ContextMenu to a HMENU, then back to a ContextMenu, and then back to a HMENU again
just convert it once right before showing it and let the UIDelegate return a new menu at at time where we don't
have to convert it back.

* WebCoreSupport/WebContextMenuClient.cpp:
(WebContextMenuClient::customizeMenu):
* WebView.cpp:
(WebView::createContextMenu):
(WebView::handleContextMenuEvent):
* WebView.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191413 268f45cc-cd09-0410-ab3c-d52691b4dbfc