We can check for buffer overflow at draw validation time, before
processing any vertex data.

BUG=angle:571

Change-Id: I4f49629b98c17ca28e25baed74cad4ae5341b20f
Reviewed-on: https://chromium-review.googlesource.com/210647Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>