Convert offscreen canvas to use FrameSinkManagerHost.

ui/compositor/compositor.h

Index: ui/compositor/compositor.h
diff --git a/ui/compositor/compositor.h b/ui/compositor/compositor.h
index fe85a4e2e150cc040e2ab2437d21f1ccc194b240..8bd8a52902d763fe1c3121868b5f68661cbf47dd 100644
--- a/ui/compositor/compositor.h
+++ b/ui/compositor/compositor.h
@@ -48,6 +48,9 @@ class LocalSurfaceId;
 class RendererSettings;
 class SurfaceManager;
 class TaskGraphRunner;
+namespace mojom {
+class DisplayCompositor;
+}
 }
 
 namespace gfx {
@@ -103,6 +106,9 @@ class COMPOSITOR_EXPORT ContextFactoryPrivate {
   // Gets the surface manager.
   virtual cc::SurfaceManager* GetSurfaceManager() = 0;
 
+  // Gets a Mojo pointer to the DisplayCompositor.
+  virtual cc::mojom::DisplayCompositor* GetDisplayCompositor() = 0;
+
   // Inform the display corresponding to this compositor if it is visible. When
   // false it does not need to produce any frames. Visibility is reset for each
   // call to CreateCompositorFrameSink.