Add SkPngChunkReader.

src/images/SkImageDecoder_libpng.cpp

Index: src/images/SkImageDecoder_libpng.cpp
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index 4101b3e4ec30ce07d57955c2f772d0b4698cdebb..b6cab3c310bb06e9b0ecad516dd81ad81abd900f 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -143,10 +143,9 @@ static void sk_seek_fn(png_structp png_ptr, png_uint_32 offset) {
 
 #ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
 static int sk_read_user_chunk(png_structp png_ptr, png_unknown_chunkp chunk) {
-    SkImageDecoder::Peeker* peeker =
-                    (SkImageDecoder::Peeker*)png_get_user_chunk_ptr(png_ptr);
-    // peek() returning true means continue decoding
-    return peeker->peek((const char*)chunk->name, chunk->data, chunk->size) ?
+    SkChunkReader* peeker = (SkChunkReader*)png_get_user_chunk_ptr(png_ptr);
+    // readChunk() returning true means continue decoding
+    return peeker->readChunk((const char*)chunk->name, chunk->data, chunk->size) ?
             1 : -1;
 }
 #endif