The spec says that DrawElements is equivalent to the instanced
call with an instance ID of zero. This patch fixes a bug in our
VertexDataManager where we would sometimes ignore the attribute divisor
for DrawElements, and stream the wrong amount of data/wrong start
offset.

BUG=angleproject:1213
TEST=dEQP-GLES3.functional.draw.draw_elements.*
TEST=dEQP-GLES3.functional.draw.draw_range_elements.*
TEST=dEQP-GLES3.functional.draw.random.*

Change-Id: I1c430a14ab3be68a24e233e9cdd1e4fd88c920a0
Reviewed-on: https://chromium-review.googlesource.com/312062
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>