[GTK] correct tabstrip drag point calculation.

Must account for discrepancy between widget->window coordinates and widget coordinates.

BUG=48774
TEST=see bug

Review URL: http://codereview.chromium.org/3003001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@52507 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed