Instead of free and allocate a descriptor set whenever buffer changes,
add it into the descriptorset cache. Every time you get the buffer, we
also look in the cache first to see if there is a descriptorset for that
buffer or not. If yes, we use the cached descriptor set instead of
allocate one.

Bug: b/159457348
Change-Id: I64c252dfe22db66ea8188415e1b384af371b82ab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2273698
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>