Testing with TSN found a race condition with RefCounted objects
(DescriptorSetLayout and PipelineLayout). Rather than add more lock
calls to protect accesses to mRefCount and mObject recommendation was to
put these caches in the ShareGroup (basically part of the context).
Locking at the GL level will ensure that two threads that share the same
context will not access the ShareGroup at the same time.

The ShareGroup also works because these layouts are not destroyed until
the context is destroyed so don't have to worry about other threads
(e.g. command processor thread) accessing them.

Bug: b/168744561
Change-Id: Icc0aa07bf4787a69572d6ec62da2f21d286232c3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2437509
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>