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

ui/gfx/buffer_types.h

Index: ui/gfx/buffer_types.h
diff --git a/ui/gfx/buffer_types.h b/ui/gfx/buffer_types.h
index 2c879056ad68369b8ae1732a94becf3f63348d80..04e9eaa8ff603d0415054e3ad13178f6698ccce7 100644
--- a/ui/gfx/buffer_types.h
+++ b/ui/gfx/buffer_types.h
@@ -32,10 +32,12 @@ enum class BufferFormat {
 // *_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.