Add alternative ambient shadow method to Android shadow sample

include/effects/SkGaussianEdgeShader.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkGaussianEdgeShader_DEFINED
+#define SkGaussianEdgeShader_DEFINED
+
+#include "SkShader.h"
+
+class SK_API SkGaussianEdgeShader {
+public:
+    /** Returns a shader that applies a Gaussian blur depending on distance to the edge
+    * Currently this is only useable with Circle and RRect shapes on the GPU backend.
+    * Raster will draw nothing.
+    */
+    static sk_sp<SkShader> Make();
+
+    SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
+};
+
+#endif