Don't limit* the height of CPB

This means we can't so height animation in CSS, but opacity-only
animation seems pleasent enough to the eye.

We can't limit height because we don't know how many item CPB will
contain if people install additional gadgets.

* It's still limited but to 9999px
Bug: 65285
Change-Id: I8d6551bccc5cbb85ff4b4dd4afb53e15dc6a163e
Merge "Don't limit* the height of CPB"