Make SkImageFilter methods const.

src/utils/SkPictureUtils.cpp

Index: src/utils/SkPictureUtils.cpp
diff --git a/src/utils/SkPictureUtils.cpp b/src/utils/SkPictureUtils.cpp
index 4af8da8ecfcc541f594053c3ec3da75061733293..92d0487c54ccfdb919336aa1e980adf620360d1f 100644
--- a/src/utils/SkPictureUtils.cpp
+++ b/src/utils/SkPictureUtils.cpp
@@ -75,9 +75,9 @@ public:
     }
     virtual void lockPixels() SK_OVERRIDE { nothing_to_do(); }
     virtual void unlockPixels() SK_OVERRIDE { nothing_to_do(); }
-    virtual bool allowImageFilter(SkImageFilter*) SK_OVERRIDE { return false; }
-    virtual bool canHandleImageFilter(SkImageFilter*) SK_OVERRIDE { return false; }
-    virtual bool filterImage(SkImageFilter*, const SkBitmap&, const SkMatrix&,
+    virtual bool allowImageFilter(const SkImageFilter*) SK_OVERRIDE { return false; }
+    virtual bool canHandleImageFilter(const SkImageFilter*) SK_OVERRIDE { return false; }
+    virtual bool filterImage(const SkImageFilter*, const SkBitmap&, const SkMatrix&,
                              SkBitmap* result, SkIPoint* offset) SK_OVERRIDE {
         return false;
     }