This enables us to use the same code for streaming client side index
arrays and for translating buffers.

Also includes a few more code cleanups.

Bug: angleproject:2786
Change-Id: Ic615d87cb50fd0acd8ab6b63ed334da4b1c40eff
Reviewed-on: https://chromium-review.googlesource.com/1188952
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>