Using FixedVector, we can remove the need for dynamic allocation in
most of the use cases. Only when we exceed a certain amount of fixed
storage do we need to spill into the heap.

Bug: angleproject:2389
Bug: chromium:829906
Change-Id: Ib5f7073b58876bfd2400bd87b743bddcee5e5dc8
Reviewed-on: https://chromium-review.googlesource.com/1002884Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>