This currently keeps a cache of every PSO compiled and does not trim
the cache or evict old members on memory pressure. This will be done
as a follow-up.

Improves the speed of the Draw Call microbenchmark 50x when using a
single state change.

Bug: angleproject:2163
Change-Id: I2cceb38ca57ae639f36a944f4571b627481b92da
Reviewed-on: https://chromium-review.googlesource.com/876954Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>