Add toString methods to SkImageFilter-derived classes

src/effects/SkTestImageFilters.cpp

Index: src/effects/SkTestImageFilters.cpp
diff --git a/src/effects/SkTestImageFilters.cpp b/src/effects/SkTestImageFilters.cpp
index be991f6f837eb8741b89ca258827b6f907e2c7e1..f0f8ae039041e3ecd6b05bd4468439b3d9af3209 100755
--- a/src/effects/SkTestImageFilters.cpp
+++ b/src/effects/SkTestImageFilters.cpp
@@ -80,3 +80,10 @@ void SkDownSampleImageFilter::flatten(SkWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);
     buffer.writeScalar(fScale);
 }
+
+#ifndef SK_IGNORE_TO_STRING
+void SkDownSampleImageFilter::toString(SkString* str) const {
+    str->appendf("SkDownSampleImageFilter: (");
+    str->append(")");
+}
+#endif