Document-relative page overlays drop some mouse events with non-zero top content...

Document-relative page overlays drop some mouse events with non-zero top content inset
https://bugs.webkit.org/show_bug.cgi?id=135871
<rdar://problem/17982275>

Reviewed by Beth Dakin.

* WebProcess/WebPage/PageOverlay.cpp:
(WebKit::PageOverlay::mouseEvent):
Convert the mouse position into document-relative coordinates; the bounds()
already are! This way, we can actually compare them without being wrong sometimes.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@172512 268f45cc-cd09-0410-ab3c-d52691b4dbfc