Share sheet doesn't attach to the window

Share sheet doesn't attach to the window
https://bugs.webkit.org/show_bug.cgi?id=138062
<rdar://problem/18767397>

Reviewed by Beth Dakin.

* UIProcess/mac/WKActionMenuController.h:
WKActionMenuController is now a NSSharingService{Picker}Delegate.

* UIProcess/mac/WKActionMenuController.mm:
(-[WKActionMenuController _defaultMenuItemsForImage]):
Set us up as the NSSharingServicePicker delegate.

(-[WKActionMenuController sharingServicePicker:delegateForSharingService:]):
Set us up as the NSSharingService delegate.

(-[WKActionMenuController sharingService:sourceWindowForShareItems:sharingContentScope:]):
Return the appropriate window to attach to.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@175190 268f45cc-cd09-0410-ab3c-d52691b4dbfc