[ios] Sets the InfobarCoordinator BadgeDelegate to nil on stop.

This crash was similar to https://crbug.com/983179, where the webState
was invalid and the badgeDelegate was trying to update the badge when a
banner had been dismissed.

In this case it seems that by the time the InfobarBanner has been presented
the webstate is no longer valid (which means the InfobarCoordinator has
been stopped), so we shouldn't try to update the badgeDelegate.

This CL nils the badgeDelegate on stop to prevent this.

Bug: 1054661
Change-Id: Ie0da80981603ede3c4a3ffa1fc5706115e5bf28b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067940
Auto-Submit: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743372}

ios/chrome/browser/ui/infobars/coordinators/infobar_coordinator.mm

@@ -89,6 +89,7 @@ const CGFloat kBannerOverlapWithOmnibox = 5.0;
 
 - (void)stop {
   _animatedFullscreenDisabler = nullptr;
+  _badgeDelegate = nil;
   _infobarDelegate = nil;
 }