[ios] Fix history coordinator |-stopWithCompletion:|

The completion handler was not properly being run in history coordinator
when the coordinator was previously stopped.

Bug: 854597
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I3a317f8776ea10452889c39a69cb257d4a8e7073
Reviewed-on: https://chromium-review.googlesource.com/1111619Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569692}

ios/chrome/browser/ui/history/history_coordinator.mm

@@ -98,11 +98,16 @@
   [self stopWithCompletion:nil];
 }
 
+// This method should always execute the |completionHandler|.
 - (void)stopWithCompletion:(ProceduralBlock)completionHandler {
-  [self.historyNavigationController
-      dismissViewControllerAnimated:YES
-                         completion:completionHandler];
-  self.historyNavigationController = nil;
+  if (self.historyNavigationController) {
+    [self.historyNavigationController
+        dismissViewControllerAnimated:YES
+                           completion:completionHandler];
+    self.historyNavigationController = nil;
+  } else if (completionHandler) {
+    completionHandler();
+  }
 }
 
 #pragma mark - HistoryLocalCommands