Implement temporary reference assignment with DisplayCompositor.

cc/ipc/mojo_compositor_frame_sink.mojom

Index: cc/ipc/mojo_compositor_frame_sink.mojom
diff --git a/cc/ipc/mojo_compositor_frame_sink.mojom b/cc/ipc/mojo_compositor_frame_sink.mojom
index aad4152e12d55ac079273ebd6fb89f28572a77dc..5d08bd958846e177dbce180a72162ed0ca8fc4e7 100644
--- a/cc/ipc/mojo_compositor_frame_sink.mojom
+++ b/cc/ipc/mojo_compositor_frame_sink.mojom
@@ -9,7 +9,7 @@ import "cc/ipc/compositor_frame.mojom";
 import "cc/ipc/copy_output_request.mojom";
 import "cc/ipc/frame_sink_id.mojom";
 import "cc/ipc/local_surface_id.mojom";
-import "cc/ipc/surface_reference.mojom";
+import "cc/ipc/surface_id.mojom";
 import "cc/ipc/returned_resource.mojom";
 import "cc/ipc/surface_sequence.mojom";
 import "ui/gfx/geometry/mojo/geometry.mojom";
@@ -74,6 +74,10 @@ interface MojoCompositorFrameSinkClient {
 // perform privilieged operations on a CompositorFrameSink such as
 // CopyOutputRequests.
 interface MojoCompositorFrameSinkPrivate {
+  // Claims this FrameSinkId will embed |surface_id| so it should own the
+  // temporary reference to |surface_id|.
+  ClaimTemporaryReference(SurfaceId surface_id);

[Tom Sepez, 2017/02/28 18:46:58]

can this fail?  how do things break if an ill-behaving client fails to call this
or unexpectedly releases it?

[kylechar, 2017/02/28 18:59:51]

MojoCompositorFrameSinkPrivate is the private interface between WS (or browser)
and GPU that is trusted. It corresponds to a MojoCompositorFrameSink interface
between the client and GPU that is untrusted.

We expect the WS to always respond correctly. If there is no temporary reference
(for example the client beat the WS by adding a reference) then this will "fail"
somewhere in the GPU but only because it's not needed. The WS doesn't need to
know if it fails, no further action is necessary.

+
   // 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.