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: de016e52bd170d2d4f2344f9bf92d50478b649e0@54851 0039d316-1c4b-4281-b951-d872f2087c98

yitian134/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,gavinp/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,adobe/chromium,ropik/chromium,adobe/chromium,ropik/chromium,ropik/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,yitian134/chromium