Remove NoPendingIO flags from buffer creations.

Now that we're keeping refs on GrBuffers during flush, we shouldn't
need to specify this flag.

Change-Id: Idbe08e84d23690d6c9e94f23f2c4507a2e965aff
Reviewed-on: https://skia-review.googlesource.com/c/188635
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>