remove unneeded readbuffer flags

- buffers are always 'cross-process'
- readbuffer is always validating

Bug:796107
Change-Id: I59614e9c29490c0b029c60d2aafe2806671bc9e1
Reviewed-on: https://skia-review.googlesource.com/90560
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
14 files changed