No-op when starting a started coordinator (ditto for stop).

ios/shared/chrome/browser/ui/coordinators/browser_coordinator.mm

Index: ios/shared/chrome/browser/ui/coordinators/browser_coordinator.mm
diff --git a/ios/shared/chrome/browser/ui/coordinators/browser_coordinator.mm b/ios/shared/chrome/browser/ui/coordinators/browser_coordinator.mm
index 338fa15ab0be6c6da17b83cbdaf8909349437c6e..0f124588a1333e6ac0dcc9c3387fcad32ad86603 100644
--- a/ios/shared/chrome/browser/ui/coordinators/browser_coordinator.mm
+++ b/ios/shared/chrome/browser/ui/coordinators/browser_coordinator.mm
@@ -39,17 +39,21 @@
 #pragma mark - Public API
 
 - (void)start {
+  if (self.started) {
+    return;
+  }
   self.started = YES;
   [self.parentCoordinator childCoordinatorDidStart:self];
 }
 
 - (void)stop {
+  if (!self.started) {
+    return;
+  }
   [self.parentCoordinator childCoordinatorWillStop:self];
   self.started = NO;
   for (BrowserCoordinator* child in self.children) {
-    if (child.started) {
-      [child stop];
-    }
+    [child stop];
   }
 }