Recomputing the stream offset is relatively cheap now after recent changes.
Remove caching it to make the code less complex.

TEST=angle_end2end_tests
BUG=angleproject:956

Change-Id: Icb9cc4e4ffd685a78b7f8a45958992a471ebb0f5
Reviewed-on: https://chromium-review.googlesource.com/262422Reviewed-by: Nicolas Capens <capn@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>