Add DisplayCompositorTest.

cc/surfaces/surface_manager.cc

Index: cc/surfaces/surface_manager.cc
diff --git a/cc/surfaces/surface_manager.cc b/cc/surfaces/surface_manager.cc
index b8b121142ff7f18f707f9dea142be0f0faa6c6ee..b95954aab2659af9264ca1d6977304f64427dfd8 100644
--- a/cc/surfaces/surface_manager.cc
+++ b/cc/surfaces/surface_manager.cc
@@ -90,6 +90,10 @@ void SurfaceManager::InvalidateFrameSinkId(const FrameSinkId& frame_sink_id) {
   GarbageCollectSurfaces();
 }
 
+const SurfaceId& SurfaceManager::GetRootSurfaceId() const {
+  return kRootSurfaceId;
+}
+
 void SurfaceManager::AddSurfaceReference(const SurfaceId& parent_id,
                                          const SurfaceId& child_id) {
   DCHECK(thread_checker_.CalledOnValidThread());