Make NinePatchPeeker inherit from SkPngChunkReader

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