This new storage implementation uses CPU memory to read back
pixel data from a texture. We use this class to implement
pack buffers for asynchronous pixel readback.

BUG=angle:511

Change-Id: Ifebd242047291af6b34c981231b69987b87d290d
Reviewed-on: https://chromium-review.googlesource.com/193243Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>