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

ios/clean/chrome/browser/ui/web_contents/web_coordinator.mm

Index: ios/clean/chrome/browser/ui/web_contents/web_coordinator.mm
diff --git a/ios/clean/chrome/browser/ui/web_contents/web_coordinator.mm b/ios/clean/chrome/browser/ui/web_contents/web_coordinator.mm
index f736febf936796e11525de871a2b6194f8631782..0248d703516213e59bad6213773a8340228785a8 100644
--- a/ios/clean/chrome/browser/ui/web_contents/web_coordinator.mm
+++ b/ios/clean/chrome/browser/ui/web_contents/web_coordinator.mm
@@ -50,9 +50,11 @@
   [self.context.baseViewController presentViewController:self.viewController
                                                 animated:self.context.animated
                                               completion:nil];
+  [super start];
 }
 
 - (void)stop {
+  [super stop];
   // PLACEHOLDER: This is how the webUsageEnabled is set to false. Find a
   // better way in the future.
   self.mediator.webState = nullptr;