Make SkImageFilter methods const.

include/effects/SkBlurImageFilter.h

 /*
  * Copyright 2011 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 SkBlurImageFilter_DEFINED
 #define SkBlurImageFilter_DEFINED
 
 #include "SkImageFilter.h"
 #include "SkSize.h"
 
 class SK_API SkBlurImageFilter : public SkImageFilter {
 public:
     SkBlurImageFilter(SkScalar sigmaX,
                       SkScalar sigmaY,
                       SkImageFilter* input = NULL,
                       const CropRect* cropRect = NULL);
     virtual void computeFastBounds(const SkRect&, SkRect*) const SK_OVERRIDE;
 
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkBlurImageFilter)
 
 protected:
     explicit SkBlurImageFilter(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&,
                                 SkIRect* dst) const SK_OVERRIDE;
 
     bool canFilterImageGPU() const SK_OVERRIDE { return true; }
     virtual bool filterImageGPU(Proxy* proxy, const SkBitmap& src, const SkMatrix& ctm,
-                                SkBitmap* result, SkIPoint* offset) SK_OVERRIDE;
+                                SkBitmap* result, SkIPoint* offset) const SK_OVERRIDE;
 
 private:
     SkSize   fSigma;
     typedef SkImageFilter INHERITED;
 };
 
 #endif