Add IndexBufferOffsetTest This test that using glDrawElements with an offset and an index buffer works correctly with or without a static copy of the buffer in D3D. BUG=510585 Change-Id: I4574a4058cc803e2ee0306e5bac52b6df6a71530 Reviewed-on: https://chromium-review.googlesource.com/288352 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>