Also cleans up GenerateWorkarounds a bit for D3D11.

BUG=angleproject:2221

Change-Id: I01667f850051045006d3439406aac3f4d094a374
Reviewed-on: https://chromium-review.googlesource.com/754915Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>