make SkRBuffer always validate

BUG=skia:6102

Change-Id: Ic9fb259b2e980d00e179340945c50492f1803a4f
Reviewed-on: https://skia-review.googlesource.com/6736
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
5 files changed