Surfaces: Allow parent to refer to child surfaces that don't exist yet.

cc/surfaces/surface_manager.h

Index: cc/surfaces/surface_manager.h
diff --git a/cc/surfaces/surface_manager.h b/cc/surfaces/surface_manager.h
index 82ba742ddc3588d214a7d504566dd46a812dbcce..b4785a246836dd4085202d496482e72856590764 100644
--- a/cc/surfaces/surface_manager.h
+++ b/cc/surfaces/surface_manager.h
@@ -38,6 +38,10 @@ class CompositorFrame;
 class Surface;
 class SurfaceFactoryClient;
 
+namespace test {
+class CompositorFrameSinkSupportTest;
+}
+
 class CC_SURFACES_EXPORT SurfaceManager {
  public:
   enum class LifetimeType {
@@ -161,6 +165,7 @@ class CC_SURFACES_EXPORT SurfaceManager {
   }
 
  private:
+  friend class test::CompositorFrameSinkSupportTest;
   friend class SurfaceManagerRefTest;
 
   using SurfaceIdSet = std::unordered_set<SurfaceId, SurfaceIdHash>;