This was causing several dEQP tests with index data to fail.

BUG=angleproject:1073
TEST=dEQP-GLES3.functional.buffer.map.read_write.render_as_index_array.*

Change-Id: Ic231a561202fbcda499b7584905b80252a7407de
Reviewed-on: https://chromium-review.googlesource.com/286772Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>