SkImageDecoder is being phased out. The SkImageDecoder::Peeker has been repurposed as SkPngChunkReader, which is shared by SkImageDecoder and SkCodec (SkImageDecoder's replacement). Make NinePatchPeeker inherit from the new class, and make its SkImageDecoder mHost optional, so we can easily switch to SkCodec. Change-Id: I74eeb909d1a4f4fe928d079857f92c142351b496