[iOS] Don't call [BrowserViewWrangler updateDeviceSharingManager] in shutdown.

Simple fix for the bug, which is caused by accessing the active web state after
the web state list has been torn down.

(cherry picked from commit 2ee3bb09e80dc73db53df12fe0b92f2e86aa462a)

Bug: 968475
Change-Id: Ib450a5ac7123d23dbb03dbdd0130a9a798fb153d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649586
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#668377}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1656251
Reviewed-by: Kariah Davis <kariahda@chromium.org>
Cr-Commit-Position: refs/branch-heads/3809@{#259}
Cr-Branched-From: d82dec1a818f378c464ba307ddd9c92133eac355-refs/heads/master@{#665002}

diff --git a/ios/chrome/browser/ui/main/browser_view_wrangler.mm b/ios/chrome/browser/ui/main/browser_view_wrangler.mm
index e810200..262ff6c 100644
--- a/ios/chrome/browser/ui/main/browser_view_wrangler.mm
+++ b/ios/chrome/browser/ui/main/browser_view_wrangler.mm

@@ -296,9 +296,10 @@
                 oldWebState:(web::WebState*)oldWebState
                     atIndex:(int)atIndex
                      reason:(int)reason {
-  if (newWebState) {
-    [self updateDeviceSharingManager];
-  }
+  if (_isShutdown)
+    return;
+
+  [self updateDeviceSharingManager];
 }
 
 #pragma mark - CRWWebStateObserver