Blink is migrating away from SkBitmaps, so we need an SkImage-based
SkImageFilter source. This is pretty much a 1-1 equivalent of
SkBitmapSource.
To avoid duplication, relocate the SkImage deserialization logic
from SkPictureData to SkReadBuffer.
R=reed@google.com,robertphillips@google.com,senorblanco@chromium.org
Review URL: https://codereview.chromium.org/1343703005
