This bug could trigger a potential extra copy when drawing with
a pack buffer (or possibly packing vertex/index data).

BUG=angle:912

Change-Id: Ifead6524322aac5bc0be79a05f4db08c8f5a3225
Reviewed-on: https://chromium-review.googlesource.com/249181Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>