This gives a very fast query time for handles that are within a fixed
range. For WebGL, where we don't allow create-on-bind, this will be
100% of the time, unless we create a very large number of resources.

It is implemented as a two-tier map - the first uses a flat array to
index into a handle buffer. The second tier uses a map for out-of-
range values.

BUG=angleproject:1458

Change-Id: I421bb3725cf523918cdfdbfaab035ad0dd3bf82d
Reviewed-on: https://chromium-review.googlesource.com/544684
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>