The buffer size did not take layer count into account.

angleproject: 2828
Change-Id: I94b9294503d785d3e727422550d7e4643eed1917
Reviewed-on: https://chromium-review.googlesource.com/c/1359618
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>