Change SkCanvasState to use inheritance.

include/utils/SkCanvasStateUtils.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCanvasStateUtils_DEFINED
 #define SkCanvasStateUtils_DEFINED
 
 #include "SkCanvas.h"
 
 class SkCanvasState;
 
 /**
- * A set of functions that are useful for copying an SkCanvas across a library
- * boundary where the Skia libraries on either side of the boundary may not be
- * version identical.  The expected usage is outline below...
+ * A set of functions that are useful for copying the state of an SkCanvas
+ * across a library boundary where the Skia library on the other side of the
+ * boundary may be newer. The expected usage is outline below...
  *
  *                          Lib Boundary
  * CaptureCanvasState(...)      |||
  *   SkCanvas --> SkCanvasState |||
  *                              ||| CreateFromCanvasState(...)
  *                              |||   SkCanvasState --> SkCanvas`
  *                              ||| Draw into SkCanvas`
  *                              ||| Unref SkCanvas`
  * ReleaseCanvasState(...)      |||
  *
  */
 namespace SkCanvasStateUtils {
     /**
      * Captures the current state of the canvas into an opaque ptr that is safe
-     * to pass between different instances of Skia (which may or may not be the
-     * same version). The function will return NULL in the event that one of the
+     * to pass to a different instance of Skia (which may be the same version,
+     * or may be newer). The function will return NULL in the event that one of the
      * following conditions are true.
      *  1) the canvas device type is not supported (currently only raster is supported)
      *  2) the canvas clip type is not supported (currently only non-AA clips are supported)
      *
      * It is recommended that the original canvas also not be used until all
      * canvases that have been created using its captured state have been dereferenced.
      *
      * Finally, it is important to note that any draw filters attached to the
      * canvas are NOT currently captured.
      *
      * @param canvas The canvas you wish to capture the current state of.
      * @return NULL or an opaque ptr that can be passed to CreateFromCanvasState
      *         to reconstruct the canvas. The caller is responsible for calling
      *         ReleaseCanvasState to free the memory associated with this state.
      */
     SK_API SkCanvasState* CaptureCanvasState(SkCanvas* canvas);
 
     /**
      * Create a new SkCanvas from the captured state of another SkCanvas. The
      * function will return NULL in the event that one of the
      * following conditions are true.
      *  1) the captured state is in an unrecognized format
      *  2) the captured canvas device type is not supported
      *
-     * @param canvas The canvas you wish to capture the current state of.
+     * @param state Opaque object created by CaptureCanvasState.
      * @return NULL or an SkCanvas* whose devices and matrix/clip state are
      *         identical to the captured canvas. The caller is responsible for
      *         calling unref on the SkCanvas.
      */
     SK_API SkCanvas* CreateFromCanvasState(const SkCanvasState* state);
 
     /**
      * Free the memory associated with the captured canvas state.  The state
      * should not be released until all SkCanvas objects created using that
-     * state have been dereferenced.
+     * state have been dereferenced. Must be called from the same library
+     * instance that created the state via CaptureCanvasState.
      *
      * @param state The captured state you wish to dispose of.
      */
     SK_API void ReleaseCanvasState(SkCanvasState* state);
 };
 
 #endif