Make SkImageFilter methods const.

include/effects/SkBitmapSource.h

 /*
  * Copyright 2012 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 SkBitmapSource_DEFINED
 #define SkBitmapSource_DEFINED
 
 #include "SkImageFilter.h"
 #include "SkBitmap.h"
 
 class SK_API SkBitmapSource : public SkImageFilter {
 public:
     explicit SkBitmapSource(const SkBitmap& bitmap);
     SkBitmapSource(const SkBitmap& bitmap, const SkRect& srcRect, const SkRect& dstRect);
     virtual void computeFastBounds(const SkRect& src, SkRect* dst) const SK_OVERRIDE;
 
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkBitmapSource)
 
 protected:
     explicit SkBitmapSource(SkReadBuffer& buffer);
     virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const SkMatrix&,
-                               SkBitmap* result, SkIPoint* offset) SK_OVERRIDE;
+                               SkBitmap* result, SkIPoint* offset) const SK_OVERRIDE;
     virtual bool onFilterBounds(const SkIRect& src, const SkMatrix& ctm, SkIRect* dst) const SK_OVERRIDE;
 
 private:
     SkBitmap fBitmap;
     SkRect   fSrcRect, fDstRect;
     typedef SkImageFilter INHERITED;
 };
 
 #endif