Make SkImageFilter methods const.

src/core/SkDeviceImageFilterProxy.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 SkDeviceImageFilterProxy_DEFINED
 #define SkDeviceImageFilterProxy_DEFINED
 
 #include "SkImageFilter.h"
 
 class SkDeviceImageFilterProxy : public SkImageFilter::Proxy {
 public:
     SkDeviceImageFilterProxy(SkBaseDevice* device) : fDevice(device) {}
 
     virtual SkBaseDevice* createDevice(int w, int h) SK_OVERRIDE {
         return fDevice->createCompatibleDevice(SkBitmap::kARGB_8888_Config,
                                                w, h, false);
     }
-    virtual bool canHandleImageFilter(SkImageFilter* filter) SK_OVERRIDE {
+    virtual bool canHandleImageFilter(const SkImageFilter* filter) SK_OVERRIDE {
         return fDevice->canHandleImageFilter(filter);
     }
-    virtual bool filterImage(SkImageFilter* filter, const SkBitmap& src,
+    virtual bool filterImage(const SkImageFilter* filter, const SkBitmap& src,
                              const SkMatrix& ctm,
                              SkBitmap* result, SkIPoint* offset) SK_OVERRIDE {
         return fDevice->filterImage(filter, src, ctm, result, offset);
     }
 
 private:
     SkBaseDevice* fDevice;
 };
 
 #endif