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..97f59f631315e16cf6c461c40e5df3be15536704 100644
--- a/ios/clean/chrome/browser/browser_coordinator+internal.h
+++ b/ios/clean/chrome/browser/browser_coordinator+internal.h
@@ -55,6 +55,12 @@
 // |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.

[rohitrao (ping after 24h), 2017/03/08 13:01:03]

We should eventually offer guidance on what to do with this method, once we
figure out the pattern =)

+- (void)childCoordinatorDidStart:(BrowserCoordinator*)coordinator;
+
+// Called when a child coordinator will stop. This is a blank template method.
+- (void)childCoordinatorWillStop:(BrowserCoordinator*)coordinator;
+
 // Methods for adding overlay coordinators.
 
 // Returns YES if the receiver will take |overlayCoordinator| as a child.