We already handle 32-bit indices, so this change enables the corresponding extension for ES 2.0.

Bug: angleproject:2902
Change-Id: Iccde1d4aac383a9c11edf851778ff315de8d4ad1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1646032Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: James Dong <dongja@google.com>