Make peekPixels() usable with raster surface snapshots

SkSurface_Raster snapshots do not lock their backing bitmaps when the
pixel ref is shared - they only lock on deep-copy.

But since for raster surfaces the pixels are always in memory, I think
it would be OK to also lock in the former case.

This allows for optimized (zero-copy) reads of raster surface snapshot
data.

R=reed@google.com

Review URL: https://codereview.chromium.org/1256993002
7 files changed