Child coordinators notify their parent upon -start and -stop.

ios/clean/chrome/browser/browser_coordinator+internal.h

Index: ios/clean/chrome/browser/browser_coordinator+internal.h
diff --git a/ios/clean/chrome/browser/browser_coordinator+internal.h b/ios/clean/chrome/browser/browser_coordinator+internal.h
index 0fdec935159fff8e00c14f63d5a188a11ae5f579..06315554a0e069006b5df9a7d672015fffd654f5 100644
--- a/ios/clean/chrome/browser/browser_coordinator+internal.h
+++ b/ios/clean/chrome/browser/browser_coordinator+internal.h
@@ -55,6 +55,16 @@
 // |coordinator| isn't a child of the receiver, this method does nothing.
 - (void)removeChildCoordinator:(BrowserCoordinator*)coordinator;
 
+// Called when a child coordinator did start. This is a blank template method.
+// Subclasses can override this method when they need to know when their
+// children start.
+- (void)childCoordinatorDidStart:(BrowserCoordinator*)childCoordinator;
+
+// Called when a child coordinator will stop. This is a blank template method.
+// Subclasses can override this method when they need to know when their
+// children start.
+- (void)childCoordinatorWillStop:(BrowserCoordinator*)childCoordinator;
+
 // Methods for adding overlay coordinators.
 
 // Returns YES if the receiver will take |overlayCoordinator| as a child.