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

ios/clean/chrome/browser/browser_coordinator.h

Index: ios/clean/chrome/browser/browser_coordinator.h
diff --git a/ios/clean/chrome/browser/browser_coordinator.h b/ios/clean/chrome/browser/browser_coordinator.h
index 6b0654307f7d1262944b5c8d76179dbb32051637..0cb5aeea081b6e24c74ee60ffb013ed0bcfb9795 100644
--- a/ios/clean/chrome/browser/browser_coordinator.h
+++ b/ios/clean/chrome/browser/browser_coordinator.h
@@ -30,15 +30,19 @@ class Browser;
 @property(nonatomic, assign) Browser* browser;
 
 // The basic lifecycle methods for coordinators are -start and -stop. These
-// are blank template methods; child classes are expected to implement them and
-// do not need to invoke the superclass methods.
+// implementations only notify the parent coordinator when this coordinator did
+// start and will stop. Child classes are expected to override and call the
+// superclass method at the end of -start and at the beginning of -stop.
+
 // Starts the user interaction managed by the receiver. Typical implementations
 // will create a view controller and then use |baseViewController| to present
-// it.
-- (void)start;
+// it. This method needs to be called at the end of the overriding
+// implementation.
+- (void)start NS_REQUIRES_SUPER;
 
-// Stops the user interaction managed by the receiver.
-- (void)stop;
+// Stops the user interaction managed by the receiver. This method needs to be
+// called at the beginning of the overriding implementation.
+- (void)stop NS_REQUIRES_SUPER;
 
 @end