The code path in the D3D11 back-end is sigificantly different for
unsigned short indices vs unsigned int. Because of the workaround we
have for the primitive restart index in D3D11, ushort rendering can
be slower.

BUG=angleproject:2229

Change-Id: I303dcc55b0314ec45508044995ba47b250cbb87d
Reviewed-on: https://chromium-review.googlesource.com/767149Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>