Use full available height when sizing percent content in cells
This was an error in
https://codereview.chromium.org/2037783002. I shouldn't be
baking in the notion of content-box a second time to cells like that: the available height to
its content is already constrained by the content box.
I can't now see any good reason for why I did that - I think was erroneously trying
to match old, incorrect behaviour in some results.
BUG=
620326
Committed:
https://crrev.com/1afb8400a1d933cd50a16947899b0d88d2d30fb0
Cr-Commit-Position: refs/heads/master@{#402748}