Add ability to extract YUV planes from SkImage

src/utils/SkRGBAToYUV.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 SkRGBAToYUV_DEFINED
+#define SkRGBAToYUV_DEFINED
+
+#include "SkPixmap.h"
+#include "SkSize.h"
+
+class SkImage;
+// Works with any image type at the moment, but in the future it may only work with raster-backed
+// images. This really should take a SkPixmap for the input, however the implementation for the
+// time being requires an image.
+bool SkRGBAToYUV(const SkImage*, const SkISize [3], void* const planes[3],
+                 const size_t rowBytes[3], SkYUVColorSpace);
+
+#endif