SkAlphaThresholdFilter.h allow flattening

Motivation:  allows this:

    #include "SkAlphaThresholdFilter.h"
    void init() {
      SkAlphaThresholdFilter::InitializeFlattenables();
    }

BUG=skia:4613

Review URL: https://codereview.chromium.org/1500373003
diff --git a/include/effects/SkAlphaThresholdFilter.h b/include/effects/SkAlphaThresholdFilter.h
index f409ee0..17521b6 100644
--- a/include/effects/SkAlphaThresholdFilter.h
+++ b/include/effects/SkAlphaThresholdFilter.h
@@ -22,6 +22,7 @@
      */
     static SkImageFilter* Create(const SkRegion& region, SkScalar innerThreshold,
                                  SkScalar outerThreshold, SkImageFilter* input = NULL);
+    SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP();
 };
 
 #endif
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index 661d0d3..b6be944 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -22,6 +22,7 @@
 
     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 @@
     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,