Add SkPngChunkReader.

include/core/SkChunkReader.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 SkChunkReader_DEFINED
+#define SkChunkReader_DEFINED
+
+#include "SkTypes.h"
+#include "SkRefCnt.h"
+
+/**
+ *  ChunkReader

[reed1, 2015/03/27 20:13:47]

ImageChunkReader, or PNGChunkReader?

It just feels like "chunk" is awfully general, and our only contemplated use at
the moment is PNG image chunks.

[scroggo, 2015/04/01 14:53:48]

Done.

+ *  Base class for optional callbacks to retrieve meta/chunk data out of an
+ *  encoded image as it is being decoded.
+ *  Used by SkImageDecoder and SkCodec.
+ */
+class SkChunkReader : public SkRefCnt {

[scroggo, 2015/03/27 19:19:59]

How's the name? Or maybe it should be ChunkParser? Or ChunkHandler? I don't have
a strong preference.

+public:
+    SK_DECLARE_INST_COUNT(SkChunkReader)
+
+    /**
+     *  This will be called by the decoder when it sees an unknown chunk.
+     *
+     *  @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 // SkChunkReader_DEFINED