This patch implements the template class ConcurrentCache which will
be used to implement all the thread-safe object caches in Dawn (e.g.
PipelineLayout, ComputePipeline and RenderPipeline).
BUG=dawn:529
TEST=dawn_unittests
Change-Id: Ia6f75191110a93285bdb23fdc1d275444d1ee866
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/67760
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
