Move PNGImageDecoder to SkCodec

SkCodec is gaining advantages over Blink's image decoders.
scroggo@ has a nice summary:

- Skia already has some features that Chromium is interested in:
  - decoding scaled versions of images
  - decoding subsets (i.e. issue 468914)
  - SIMD optimizations for writing pixels
- In general, sharing code means that new features/bug fixes in Android
  benefit Chromium and vice versa
- Skia's API is designed to allow the client to handle caching, so that
  the client that knows the bigger picture can make caching decisions in
  a more informed way than ImageDecoder can.