Add SkPngChunkReader.

include/core/SkPngChunkReader.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkPngChunkReader_DEFINED
+#define SkPngChunkReader_DEFINED
+
+#include "SkTypes.h"
+#include "SkRefCnt.h"
+
+/**
+ *  SkPngChunkReader
+ *
+ *  Base class for optional callbacks to retrieve meta/chunk data out of a PNG
+ *  encoded image as it is being decoded.
+ *  Used by SkImageDecoder and SkCodec.
+ */
+class SkPngChunkReader : public SkRefCnt {
+public:
+    /**
+     *  This will be called by the decoder when it sees an unknown chunk.
+     *
+     *  Use by SkCodec:
+     *  Depending on the location of the unknown chunks, this callback may be
+     *  called by
+     *      - the factory (NewFromStream/NewFromData)
+     *      - getPixels
+     *      - startScanlineDecode
+     *      - the first call to getScanlines/skipScanlines
+     *  The callback may be called from a different thread (e.g. if the SkCodec
+     *  is passed to another thread), and it may be called multiple times, if
+     *  the SkCodec is used multiple times.
+     *
+     *  @param tag Name for this type of chunk.
+     *  @param data Data to be interpreted by the subclass.
+     *  @param length Number of bytes of data in the chunk.
+     *  @return true to continue decoding, or false to indicate an error, which
+     *      will cause the decoder to not return the image.
+     */
+    virtual bool readChunk(const char tag[], const void* data, size_t length) = 0;
+};
+#endif // SkPngChunkReader_DEFINED