Different drivers handle overflow differently.  The overflow case is not
defined in the spec.

BUG=angleproject:1044

Change-Id: I9e3883af147c6c1a4fcdcddea3ff70c62f859c56
Reviewed-on: https://chromium-review.googlesource.com/282540Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>