Make SkImageFilter methods const.

src/core/SkDeviceImageFilterProxy.h

Index: src/core/SkDeviceImageFilterProxy.h
diff --git a/src/core/SkDeviceImageFilterProxy.h b/src/core/SkDeviceImageFilterProxy.h
index 03fcb68125a71a79d5217700461c1a1c03f5129e..800e42c1f62d0ad743ac6cc583582b43af6adbd0 100644
--- a/src/core/SkDeviceImageFilterProxy.h
+++ b/src/core/SkDeviceImageFilterProxy.h
@@ -18,10 +18,10 @@ public:
         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);