Don't flush on read/write pixels unless necessary

BUG=skia:2889

Committed: https://skia.googlesource.com/skia/+/150723b9298772a5096bec7acd2999c5c9d66239

R=robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/586073002
5 files changed