We were using the new, larger size for our buffer data copy step,
instead of the old, smaller size. This bug was causing a crash on
older nVidia drivers during normal browser usage.

Reproducible with the index-validation-large-buffer WebGL test.

BUG=angle:667
BUG=384420

Change-Id: I98ee893e0d8ba0bfc9adfe5a338da9b940248879
Reviewed-on: https://chromium-review.googlesource.com/203776Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>