The system memory storage is needed to back UBOs larger than the max
constant buffer size in D3D11 Windows 7. Because readback from UBOs
can be tricky, the system memory storage keeps the canonical copy of
the data in this case.

We can also extend this to add a workaround to fix the outstanding
failure cases of UBOs on Intel.

BUG=chromium:660670

Change-Id: Ia3119d3064d10c4262def4c5a967972b4de2d10f
Reviewed-on: https://chromium-review.googlesource.com/405367Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>