This will be useful for the Vulkan pipeline state cache. This cache
wants to pack the pipeline description very carefully into a specific
number of bits, using the "alignas" keyword. Using this keyword
requires an aligned allocator, which we can lift from Chromium's
base/memory submodule.

Also includes a unittest copied from Chrome.

Bug: angleproject:2163
Change-Id: I25976be4610636db3f43552bba23d823f5a49a24
Reviewed-on: https://chromium-review.googlesource.com/837944Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>