Remove SkImageGenerator pieces only for SkCodec.

Follow up to the split between SkImageGenerator and SkCodec. Now that
SkCodec does not inherit from SkImageGenerator, SkImageGenerator no
longer needs Options or Result, which were added for SkCodec. Remove
them, but keep them behind a flag, since Chromium has its own
subclasses of SkImageGenerator which assume the old signature for
onGetPixels.

Review URL: https://codereview.chromium.org/1226023003
7 files changed