SkAlphaThresholdFilter allow flattening

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index 661d0d362ad2eed7ca3ce72b991d8f34d7ff6a19..b6be944220d25678ce719444422b26ba82500873 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -22,6 +22,7 @@ public:
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkAlphaThresholdFilterImpl)
+    friend void SkAlphaThresholdFilter::InitializeFlattenables();
 
 protected:
     void flatten(SkWriteBuffer&) const override;
@@ -40,6 +41,11 @@ private:
     typedef SkImageFilter INHERITED;
 };
 
+SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkAlphaThresholdFilter)
+    SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkAlphaThresholdFilterImpl)
+SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
+
+
 SkImageFilter* SkAlphaThresholdFilter::Create(const SkRegion& region,
                                               SkScalar innerThreshold,
                                               SkScalar outerThreshold,