Make SkPixelRef::isLocked() debug-only, remove related dead code.

DM's okay locally with no diffs, no failures.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/8e65712486c66108677a9b0a55ad3e7ca94db555

Review URL: https://codereview.chromium.org/940083002
5 files changed