adding new GM to demostrate new shadows

src/core/SkShadowShader.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 SkShadowShader_DEFINED
+#define SkShadowShader_DEFINED
+
+class SkLights;
+class SkShader;
+
+class SK_API SkShadowShader {
+public:
+    /** This shader combines the diffuse color in 'diffuseShader' with the shadows
+     *  determined by the 'povDepthShader' and the shadow maps stored in each of the
+     *  lights in 'lights'
+     */
+    static sk_sp<SkShader> Make(sk_sp<SkShader> povDepthShader,
+                                sk_sp<SkShader> diffuseShader,
+                                sk_sp<SkLights> lights);
+
+    SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
+};
+
+#endif