Add gpu implementation of OverdrawXfermode

src/utils/debugger/SkOverdrawMode.h

Index: src/utils/debugger/SkOverdrawMode.h
diff --git a/src/utils/debugger/SkOverdrawMode.h b/src/utils/debugger/SkOverdrawMode.h
new file mode 100644
index 0000000000000000000000000000000000000000..dba7635f6a32a05a652553d3f6d1ebdae1d974da
--- /dev/null
+++ b/src/utils/debugger/SkOverdrawMode.h
@@ -0,0 +1,25 @@
+/*
+ * 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 SkOverdrawMode_DEFINED
+#define SkOverdrawMode_DEFINED
+
+#include "SkFlattenable.h"
+
+class SkXfermode;
+
+class SkOverdrawMode {
+public:
+    static SkXfermode* Create();
+
+    SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP();
+
+private:
+    SkOverdrawMode(); // can't be instantiated
+};
+
+#endif