Make SkCodec support peek() and read()

src/codec/SkJpegCodec.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 SkJpegCodec_DEFINED
 #define SkJpegCodec_DEFINED
 
 #include "SkCodec.h"
 #include "SkImageInfo.h"
 #include "SkJpegDecoderMgr.h"
 #include "SkJpegUtility_codec.h"
 #include "SkStream.h"
 
 extern "C" {
     #include "jpeglib.h"
 }
 
 /*
  *
  * This class implements the decoding for jpeg images
  *
  */
 class SkJpegCodec : public SkCodec {
 public:
-
-    /*
-     * Checks the start of the stream to see if the image is a jpeg
-     * Does not take ownership of the stream
-     */
-    static bool IsJpeg(SkStream*);
+    static bool IsJpeg(const void*, size_t);
 
     /*
      * Assumes IsJpeg was called and returned true
      * Creates a jpeg decoder
      * Takes ownership of the stream
      */
     static SkCodec* NewFromStream(SkStream*);
 
 protected:
 
@@ skipping 73 matching lines @@
 
     // scanline decoding
     SkAutoMalloc               fStorage;    // Only used if sampling is needed
     uint8_t*                   fSrcRow;     // Only used if sampling is needed
     SkAutoTDelete<SkSwizzler>  fSwizzler;
     
     typedef SkCodec INHERITED;
 };
 
 #endif