GitLab

Layer serves no purpose except keeping track of whether there are any
pending readback requests. It used to be used for actual readback
requests, but requests have been moved to surfaces when they were
introduced.

Remove it and replace it with a counter instead.

Change-Id: I37eafbe462672703805440001346de485a03c30e
Reviewed-on: https://chromium-review.googlesource.com/1001612Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549222}