The descriptor pool was initialized with the number of atomic counter
buffers, but we always use 8 storage buffers for atomic counter buffer
emulation regardless of how many atomic counter buffers there.

This change fixes the descriptor pool initialization by requesting 8
storage buffer descriptors if there are any atomic counter buffers.

Bug: angleproject:4199
Change-Id: Ide58c013cba8ac46228dba795a82304b97ee83ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1959489
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>