Make the struct passed to SkAndroidCodec::getAndroidPixels const. This
matches SkCodec, and makes sense, since it is not used as an output.
Brought up in crrev.com/1417583009
Review URL: https://codereview.chromium.org/1411693005
We will implement this API using SkCodecs.
SkAndroidCodecs will be used to implement the
BitmapRegionDecoder Java API (and possibly
BitmapFactory).
BUG=skia:
Review URL: https://codereview.chromium.org/1406223002