[iOS] Fix crash on app exit in BrowserViewWrangler

After the switch to use WebStateListObserver instead of TabModelObserver,
BrowserViewWrangler will get a notification that the active webState is
changing when the app quits (the active webState is changing to nullptr).
This in turn causes a call to -updateDeviceSharingManager, which explodes
when there's no active WebState.

The fix is to not call -updateDeviceSharingManager with a nullptr new
active web state.


Bug: 968475
Change-Id: I78b25c627d311d2e2a3f152e8cd8f663161962bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645230
Auto-Submit: Mark Cogan <marq@chromium.org>
Reviewed-by: David Jean <djean@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666270}

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

@@ -296,7 +296,9 @@
                 oldWebState:(web::WebState*)oldWebState
                     atIndex:(int)atIndex
                      reason:(int)reason {
-  [self updateDeviceSharingManager];
+  if (newWebState) {
+    [self updateDeviceSharingManager];
+  }
 }
 
 #pragma mark - CRWWebStateObserver