[iOS] Don't attempt to present UI for new request if already presenting.

When a navigation occurs, all OverlayRequests are cancelled, which
triggers the dismissal of any presented overlay UI.  The request queue
is empty at this point, so if the newly-loaded page attempts to present
a dialog on load, it's possible that OverlayPresenterImpl will try to
show this dialog before the cancelled dialog's UI is finished being
dismissed.  This CL adds a check to prevent attempting to present if
already presenting.  If a cancelled overlay is being dismissed, the new
request's UI will be presented upon the completion of the dismissal.

Bug: 990966
Change-Id: Iee7e735b53c9844666d6ea7cc48168513d768f0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1736182Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684217}

ios/chrome/browser/overlays/overlay_presenter_impl.mm

@@ -279,10 +279,12 @@ void OverlayPresenterImpl::BrowserDestroyed(Browser* browser) {
 
 void OverlayPresenterImpl::RequestAddedToQueue(OverlayRequestQueueImpl* queue,
                                                OverlayRequest* request) {
-  // If |queue| is active and the added request is the front request, trigger
-  // the UI presentation for that request.
-  if (queue == GetActiveQueue() && request == queue->front_request())
+  // If |queue| is active, the added request is frontmost, and an overlay is not
+  // currently being presented, trigger the UI presentation for that request.
+  if (queue == GetActiveQueue() && request == queue->front_request() &&
+      !presenting_) {
     PresentOverlayForActiveRequest();
+  }
 }
 
 void OverlayPresenterImpl::QueuedRequestCancelled(