Stop the toolbar when the BrowserState is destroyed

Bug: none
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I05511ee9479c12153ac69b4c794c769d73c0f329
Reviewed-on: https://chromium-review.googlesource.com/848833Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526709}

ios/chrome/browser/ui/toolbar/toolbar_adapter.mm

@@ -56,7 +56,7 @@
 #pragma mark - Abstract WebToolbar
 
 - (void)browserStateDestroyed {
-  return;
+  [self.toolbarCoordinator stop];
 }
 
 - (void)updateToolbarState {