Speculative fix for DownloadManagerViewController callback crash.

This crash does not have steps to reproduce and can occur if Download
Task in coordinator is null or invalid. This change gracefully handles
the case when task became null after coordinator was stopped.

Setting view controller delegate to nil will prevent callbacks
from view controller which is being dismissed.

Bug: 824758
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I44099adf5604dba25e937670312f7fff5af2f56d
Reviewed-on: https://chromium-review.googlesource.com/975666Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545161}

ios/chrome/browser/ui/download/download_manager_coordinator.mm

@@ -120,6 +120,8 @@ class UnopenedDownloadsTracker : public web::DownloadTaskObserver {
 - (void)stop {
   if (_viewController) {
     [self.presenter dismissAnimated:YES];
+    // Prevent delegate callbacks for stopped coordinator.
+    _viewController.delegate = nil;
     _viewController = nil;
   }
   [_confirmationDialog dismissViewControllerAnimated:YES completion:nil];