IndexRange uses size_t values but DrawCallParams::mFirstVertex is a GLint. This
makes sure all casting is safe.

BUG=864528

Change-Id: Iaa95019615af4d3f79b12bbcb0df13b44cb54339
Reviewed-on: https://chromium-review.googlesource.com/1140898Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>