Using the same policy as for system memory storage, we release
the staging buffer if it has been unmodified for several uses.

We could further extend this technique to other storage buffers
using a nearly identical policy. We might need to mitigate the
cost of iterating over all the storages of all the buffers.

BUG=angleproject:516

Change-Id: Ib24ef03cf4184ef6f0c24f83347538df82de5f58
Reviewed-on: https://chromium-review.googlesource.com/387200Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>