D3D11 DepthBias is an integer corresponding directly to GL polygon offset units.

BUG=371604

Change-Id: I20a126bd9c6298efff95f3edbf1babc0dd495b18
Reviewed-on: https://chromium-review.googlesource.com/199351Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Nicolas Capens <nicolascapens@chromium.org>