D3D11 handles primitive restart similarly to OpenGL's fixed index,
with a couple differences. It can't be toggled off, so we need to
restrict the max element index (handled in a prior patch), and
for smaller buffer types we need to rewrite the index data.

BUG=angleproject:597

Change-Id: Ib890ce9b3f5511784138ea3953a384b1c483ca9e
Reviewed-on: https://chromium-review.googlesource.com/309639Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>