Consolidate SkCodec functions for handling rewind

include/codec/SkCodec.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 SkCodec_DEFINED
 #define SkCodec_DEFINED
 
@@ skipping 215 matching lines @@
                                void* pixels, size_t rowBytes, const Options&,
                                SkPMColor ctable[], int* ctableCount) = 0;
 
     virtual bool onGetValidSubset(SkIRect* /* desiredSubset */) const {
         // By default, subsets are not supported.
         return false;
     }
 
     virtual bool onReallyHasAlpha() const { return false; }
 
-    enum RewindState {
-        kRewound_RewindState,
-        kNoRewindNecessary_RewindState,
-        kCouldNotRewind_RewindState
-    };
     /**
      *  If the stream was previously read, attempt to rewind.
-     *  @returns:
-     *      kRewound if the stream needed to be rewound, and the
-     *               rewind succeeded.
-     *      kNoRewindNecessary if the stream did not need to be
-     *                         rewound.
-     *      kCouldNotRewind if the stream needed to be rewound, and
-     *                      rewind failed.
+     *
+     *  If the stream needed to be rewound, call onRewind.
+     *  @returns true if the codec is at the right position and can be used.
+     *      false if there was a failure to rewind.
      *
      *  Subclasses MUST call this function before reading the stream (e.g. in
      *  onGetPixels). If it returns false, onGetPixels should return
      *  kCouldNotRewind.
      */
-    RewindState SK_WARN_UNUSED_RESULT rewindIfNeeded();
+    bool SK_WARN_UNUSED_RESULT rewindIfNeeded();
+
+    /**
+     *  Called by rewindIfNeeded, if the stream needed to be rewound.
+     *
+     *  Subclasses should do any set up needed after a rewind.
+     */
+    virtual bool onRewind() {
+        return true;
+    }
 
     /**
      * Get method for the input stream
      */
     SkStream* stream() {
         return fStream.get();
     }
 
 private:
     const SkImageInfo       fInfo;
     SkAutoTDelete<SkStream> fStream;
     bool                    fNeedsRewind;
 };
 #endif // SkCodec_DEFINED