This makes it consistent with DynamicDescriptorPool, and gives a bit
more precise definition.

Bug: angleproject:2318
Change-Id: I8953113165ebe2d0dcfc0fc923d94280180442ce
Reviewed-on: https://chromium-review.googlesource.com/985199
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>