Centralize FrameSinkId allocation in one place

ui/compositor/test/in_process_context_factory.cc

Index: ui/compositor/test/in_process_context_factory.cc
diff --git a/ui/compositor/test/in_process_context_factory.cc b/ui/compositor/test/in_process_context_factory.cc
index 0a5be65e9fa2348b8cd73284239acb861ecfc957..1cd0e712871358f6117cc674e1a8c8c5f4dd2b88 100644
--- a/ui/compositor/test/in_process_context_factory.cc
+++ b/ui/compositor/test/in_process_context_factory.cc
@@ -129,7 +129,9 @@ struct InProcessContextFactory::PerCompositorData {
 InProcessContextFactory::InProcessContextFactory(
     bool context_factory_for_test,
     cc::SurfaceManager* surface_manager)
-    : next_surface_sink_id_(1u),
+    : frame_sink_id_allocator_(
+          0 /* client_id, not to conflict with the client_id
+                                   generated from RenderWidgetHostImpl */),
       use_test_surface_(true),
       context_factory_for_test_(context_factory_for_test),
       surface_manager_(surface_manager) {
@@ -285,12 +287,7 @@ cc::TaskGraphRunner* InProcessContextFactory::GetTaskGraphRunner() {
 }
 
 cc::FrameSinkId InProcessContextFactory::AllocateFrameSinkId() {
-  // The FrameSinkId generated here must be unique with
-  // RenderWidgetHostViewAura's
-  // and RenderWidgetHostViewMac's FrameSinkId allocation.
-  // TODO(crbug.com/685777): Centralize allocation in one place for easier
-  // maintenance.
-  return cc::FrameSinkId(0, next_surface_sink_id_++);
+  return frame_sink_id_allocator_.NextFrameSinkId();
 }
 
 cc::SurfaceManager* InProcessContextFactory::GetSurfaceManager() {