We now allocate and release all buffer resources from a single place,
allowing us to allocate junk memory, clear on init, and track the
currently allocated buffer memory.

BUG=angleproject:2034

Change-Id: Id60b63a7f77bc369dfc494a7587ab06c8d34a8e2
Reviewed-on: https://chromium-review.googlesource.com/503253Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>