Make SkImageFilter methods const.

gm/imagefiltersbase.cpp

Index: gm/imagefiltersbase.cpp
diff --git a/gm/imagefiltersbase.cpp b/gm/imagefiltersbase.cpp
index 109be6a1e036ed839ee826173261aeb69fbda1f4..4794b02699c30ed650a13da82e22b2be5ed14bb4 100644
--- a/gm/imagefiltersbase.cpp
+++ b/gm/imagefiltersbase.cpp
@@ -23,7 +23,7 @@ public:
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(FailImageFilter)
 protected:
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const SkMatrix&,
-                               SkBitmap* result, SkIPoint* offset) {
+                               SkBitmap* result, SkIPoint* offset) const {
         return false;
     }
 
@@ -46,7 +46,7 @@ public:
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(IdentityImageFilter)
 protected:
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const SkMatrix&,
-                               SkBitmap* result, SkIPoint* offset) {
+                               SkBitmap* result, SkIPoint* offset) const {
         *result = src;
         return true;
     }