[iOS] Stop the signin interaction coordinator in stopChromeMain

The Sign-in interaction coordinator needs to cancel when
-[MainController stopChromeMain] is called.

Bug: 1002834
Change-Id: Ideb74b457c983203508757863da6a7ce9afad075
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807213Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697129}

ios/chrome/app/main_controller.mm

@@ -964,6 +964,9 @@ enum class EnterTabSwitcherSnapshotResult {
 
 - (void)stopChromeMain {
   // The UI should be stopped before the models they observe are stopped.
+  [self.signinInteractionCoordinator cancel];
+  self.signinInteractionCoordinator = nil;
+
   [_mainCoordinator stop];
   _mainCoordinator = nil;