Add a completion handler call back in openSelectedTabInMode

completion should be called in every branch of the function, and it was not in this one,
which is a bug. Separated out from CL: https://chromium-review.googlesource.com/c/chromium/src/+/2236243
so that in case of reverting, this change can stay.

Change-Id: If1cf0522b492e8c240253e89432f16d21916da70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2320133
Commit-Queue: Andy Lu <andyhylu@google.com>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#792327}

ios/chrome/browser/ui/main/scene_controller.mm

@@ -1587,6 +1587,13 @@ const char kMultiWindowOpenInNewWindowHistogram[] =
           dismissWithNewTabAnimationToBrowser:targetInterface.browser
                             withUrlLoadParams:urlLoadParams
                                       atIndex:tabIndex];
+      // In this particular usage, there should be no postOpeningAction,
+      // as triggering voice search while there are multiple windows opened is probably
+      // a bad idea both technically and as a user experience.
+      // It should be the caller duty to not set a completion if they don't need it.
+      if (completion) {
+        completion();
+      }
     }
   } else {
     if (!self.currentInterface.viewController.presentedViewController) {