Revert of "Converted LayerImpl::bounds() to return SizeF."

Seems this was more contentious than originally anticipated. Reverting
this until we have consensus on how to go forward.

[email protected],[email protected],[email protected]
BUG=

Review URL: https://codereview.chromium.org/637913003

Cr-Commit-Position: refs/heads/master@{#298710}
26 files changed