Fix debug build GTK assert in TabContentsDragSource::OnDragBegin().
Fix debug build GTK assert in TabContentsDragSource::OnDragBegin().

The assert is:
Gtk: gtk_widget_set_colormap: assertion `!gtk_widget_get_realized
(widget)' failed.

BUG=none
TEST=Navigate to google.com and drag the logo twice.

Review URL: http://codereview.chromium.org/3086011

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@54851 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser