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Need to have the GL context current before deleting the buffers. Since
the context was destoryed before the buffers due to declaration ordering
in the class definition, the context was lost before the buffers were
destroyed.

Also rename the overlay array to signify that it is an array.

Fixed presubmit warning to use base::size over arraysize.

BUG=NONE
TEST=Ran ozone_demo and verified no GL errors are seen on console.

Change-Id: I8e8da2918fe7c1f2cb5cf16bcebddfd453b315b3
Reviewed-on: https://chromium-review.googlesource.com/1039988Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555452}