This most strongly affects the Vulkan back-end which uses a hash map
to cache VkPipelines. It also should speed up some parts of the D3D11
back-end.

Bug: angleproject:2522
Change-Id: I10801654f043abce17339c30d7bf62bc8644e49c
Reviewed-on: https://chromium-review.googlesource.com/c/1316889
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>