add safemath version of readbuffer->skip

Bug: skia:
Change-Id: I6a5d24569cc1bd93bd190d81e3375993da9c56f0
Reviewed-on: https://skia-review.googlesource.com/88581
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
5 files changed