Type confusion on the index buffer. It should be unsigned shorts. Fixes
a crash on iOS.

Bug: angleproject:4992
Bug: angleproject:5417
Change-Id: I18179a89dd81fff2582636496ea9684e432f4400
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2601162Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>