This allows us to not store std::pairs to identify SRVs. Instead
we can release the cached SRVs directly when we re-create the
native buffer.

BUG=angleproject:1513

Change-Id: I6f6d5edc659aa843c9439b97661073415f1a772b
Reviewed-on: https://chromium-review.googlesource.com/385818
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>