A more flexible representation of a buffer storage implementation
allows us to represent storage for D3D11 buffers, and also pack
buffer as CPU memory.

Refactoring patch only.

BUG=angle:511

Change-Id: Ie123099f6217bec6edc98594e49333d16f006a0b
Reviewed-on: https://chromium-review.googlesource.com/191033Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>