gtk: Initialize |background_offset_x_|.  When rendering a dragged tab, we weren't hitting the codepath that initializes this variable, leading to a UMR that caused glitchy rendering.
gtk: Initialize |background_offset_x_|.  When rendering a dragged tab, we weren't hitting the codepath that initializes this variable, leading to a UMR that caused glitchy rendering.

BUG=20157
TEST=Drag a tab.  The rendering should not be glitchy.
Review URL: http://codereview.chromium.org/176035

git-svn-id: http://src.chromium.org/svn/trunk/src@24862 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 0a853e5b65b4398e29a17c8d2429998e8b405df7
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