ui: Revert gfx::BufferUsage::SCANOUT to gfx::BufferUsage::GPU_READ_WRITE change.

ui/gfx/buffer_types.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GFX_BUFFER_TYPES_H_
 #define UI_GFX_BUFFER_TYPES_H_
 
 namespace gfx {
 
 // The format needs to be taken into account when mapping a buffer into the
@@ skipping 14 matching lines @@
   YUV_420_BIPLANAR,
   UYVY_422,
 
   LAST = UYVY_422
 };
 
 // The usage mode affects how a buffer can be used. Only buffers created with
 // *_CPU_READ_WRITE_* can be mapped into the client's address space and accessed
 // by the CPU. *_CPU_READ_WRITE_PERSISTENT adds the additional condition that
 // successive Map() calls (with Unmap() calls between) will return a pointer to
-// the same memory contents.
+// the same memory contents. SCANOUT implies GPU_READ_WRITE.
+// TODO(reveman): Add GPU_READ_WRITE for use-cases where SCANOUT is not
+// required.
 enum class BufferUsage {
   GPU_READ,
-  GPU_READ_WRITE,
+  SCANOUT,
   GPU_READ_CPU_READ_WRITE,
   // TODO(reveman): Merge this with GPU_READ_CPU_READ_WRITE when SurfaceTexture
   // backed buffers are single buffered and support it.
   GPU_READ_CPU_READ_WRITE_PERSISTENT,
 
   LAST = GPU_READ_CPU_READ_WRITE_PERSISTENT
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_BUFFER_TYPES_H_