improve indication that open tab in background opened tab in overflow area.
instead of flashing the "all tabs" button, provide a smoother animation.
note, this will not work on mac trunk (but it will work on the mac branch)
due to bug #346738 (aka #325296), so for now, there is no animation on the mac.
r=mconnor
git-svn-id: svn://10.0.0.236/trunk@206245 18797224-902f-48f8-a5cc-f745e15eee43
635ea85608
