We had a bug where we would require "stride * primCount" bytes when
drawing from a buffer, when the last element would not require the
full stride. The correct expression: "stride * (primCount-1) + size.

BUG=angleproject:1086
TEST=dEQP-GLES3.functional.vertex_arrays.single_attribute.strides.*

Change-Id: I7e2897c9d18b0e9849289d033f0ae2573237e4e5
Reviewed-on: https://chromium-review.googlesource.com/287320Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>