Move FrameSink hierarchy registration to DisplayCompositor interface

cc/ipc/mojo_compositor_frame_sink.mojom

 // Copyright 2016 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.
 
 module cc.mojom;
 
 import "cc/ipc/begin_frame_args.mojom";
 import "cc/ipc/compositor_frame.mojom";
 import "cc/ipc/copy_output_request.mojom";
 import "cc/ipc/frame_sink_id.mojom";
@@ skipping 52 matching lines @@
   // Notification for the client to generate a CompositorFrame.
   OnBeginFrame(BeginFrameArgs args);
 
   // Returns resources sent to SubmitCompositorFrame to be reused or freed.
   ReclaimResources(ReturnedResourceArray resources);
 
   // Called when surface is being scheduled for a draw.
   WillDrawSurface(LocalSurfaceId local_surface_id, gfx.mojom.Rect damage_rect);
 };
 
-// MojoCompositorFrameSinkPrivate is used by the display compositor host to set
-// up BeginFrameSource hierarchies.  This API lives in SurfaceManager but is
-// called from here in order to ensure that hierarchy registration does not race
-// CompositorFrameSink creation.
+// MojoCompositorFrameSinkPrivate is used by the display compositor host to
+// perform privilieged operations on a CompositorFrameSink such as
+// CopyOutputRequests.
 interface MojoCompositorFrameSinkPrivate {
-  AddChildFrameSink(FrameSinkId child_frame_sink_id);
-  RemoveChildFrameSink(FrameSinkId child_frame_sink_id);
-
   // Requests that the renderer send back a copy of the surface that this
   // CompositorFrameSink submits to. The result can be in form of a bitmap
   // or a texture. See cc::CopyOutputRequest.
   RequestCopyOfSurface(cc.mojom.CopyOutputRequest request);
 };