Since the gl::Buffer doesn't know about its data, the index range needs to
be computed on the raw indices instead of through the buffer.

Change-Id: I3f53822033a1a25bf5fae9132ee2ceb312eaa283
Reviewed-on: https://chromium-review.googlesource.com/270498Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>