[WK2] Take TopContentInset into account when sending dictionary lookup point to Plugi...

[WK2] Take TopContentInset into account when sending dictionary lookup point to PluginView
https://bugs.webkit.org/show_bug.cgi?id=134624
<rdar://problem/17222041>

Reviewed by Sam Weinig.

* WebProcess/Plugins/PDF/PDFPlugin.mm:
(WebKit::PDFPlugin::performDictionaryLookupAtLocation):
Perform the same coordinate conversion we use for mouse events, which happens
to take the top content inset into account.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@170810 268f45cc-cd09-0410-ab3c-d52691b4dbfc