Make SkImageFilter methods const.

include/effects/SkXfermodeImageFilter.h

 /*
  * Copyright 2013 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkXfermodeImageFilter_DEFINED
 #define SkXfermodeImageFilter_DEFINED
 
@@ skipping 14 matching lines @@
                           SkImageFilter* foreground = NULL, const CropRect* cropRect = NULL);
 
     virtual ~SkXfermodeImageFilter();
 
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkXfermodeImageFilter)
 
     virtual bool onFilterImage(Proxy* proxy,
                                const SkBitmap& src,
                                const SkMatrix& ctm,
                                SkBitmap* dst,
-                               SkIPoint* offset) SK_OVERRIDE;
+                               SkIPoint* offset) const SK_OVERRIDE;
 #if SK_SUPPORT_GPU
     virtual bool canFilterImageGPU() const SK_OVERRIDE { return !cropRectIsSet(); }
     virtual bool filterImageGPU(Proxy* proxy, const SkBitmap& src, const SkMatrix& ctm,
-                                SkBitmap* result, SkIPoint* offset) SK_OVERRIDE;
+                                SkBitmap* result, SkIPoint* offset) const SK_OVERRIDE;
 #endif
 
 protected:
     explicit SkXfermodeImageFilter(SkReadBuffer& buffer);
     virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
 
 private:
     SkXfermode* fMode;
     typedef SkImageFilter INHERITED;
 };
 
 #endif