The change to initialize a constant buffer immediately after
calling BufferData in D3D11 led to a bug where we would map
the UBO for writing with the wrong map bits. Fix this by
using the same map method as the rest of the code.

The D3D11 runtime seems to allow arbitrarily large constant buffers
on Windows 10, but not Windows 7. Thus this CL also fixes a bug in
our constant buffer size clamping to not copy more than the available
buffer size for uniform buffers.

BUG=chromium:651493

Change-Id: I876767691d02db90ecb08a8fa78199f03339a35e
Reviewed-on: https://chromium-review.googlesource.com/391167Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>