In D3D11, map-discard is reserved for DYNAMIC usage buffer,
which are resources that live on the GPU. Staging buffers
don't have the notion of map-discard, and using them with
discard gives a D3D11 error.

This patch fixes crashes in dEQP buffer.map.write.invalidate.

BUG=angle:587

Change-Id: Ie225e72d4e226c69e73826c9fb67bbb940bbc466
Reviewed-on: https://chromium-review.googlesource.com/190072Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>