Gtk: fix context menu positioning, take 2.
Gtk: fix context menu positioning, take 2.

Find correct screen (based on current position of mouse pointer) and monitor.

BUG=27800
Review URL: http://codereview.chromium.org/399101

git-svn-id: http://src.chromium.org/svn/trunk/src@41894 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 080d710e8928548f9603f3666f44543df8586dfd
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser