This optimized vector keeps an initial fixed size storage but has
unbounded growth like a normal std::vector. It operates like a
FixedVector initially but then switches the storage to an allocated
pool when the element count exceeds the array limit.

This gives fast performance in the small case since no dynamic
allocation is needed. It also handles the "slow" big case.

Bug: angleproject:2763
Change-Id: I9c002b205bd4ac9fc171d1fdc42bc8ea5fe8dabe
Reviewed-on: https://chromium-review.googlesource.com/1227794Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>