Mid-air collision: fix constness for onFilterImage in new test.

tests/ImageFilterTest.cpp

Index: tests/ImageFilterTest.cpp
diff --git a/tests/ImageFilterTest.cpp b/tests/ImageFilterTest.cpp
index a1d7a50f05728cd34fcc53638465b29852e2df0c..317a877d607abb750c8ac30103e8bfb7d0da3c22 100644
--- a/tests/ImageFilterTest.cpp
+++ b/tests/ImageFilterTest.cpp
@@ -44,7 +44,7 @@ public:
     }
 
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const SkMatrix& ctm,
-                               SkBitmap* result, SkIPoint* offset) SK_OVERRIDE {
+                               SkBitmap* result, SkIPoint* offset) const SK_OVERRIDE {
         REPORTER_ASSERT(fReporter, ctm == fExpectedMatrix);
         return true;
     }