In GLES, the start vertex only applies to non-instanced vertex
attributes.

BUG=angle:864
BUG=447140

Change-Id: Idd2afbfbd4c2e76e06b2704cc002fae26b353109
Reviewed-on: https://chromium-review.googlesource.com/239843Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/241870