[ios] Early return if webState is not valid

Sometimes [ToolbarMediator updateShareMenuForWebState:]
is passed a null WebState, so do nothing if that is the case.

Bug: 
Change-Id: Id083687df59199489a80ca21a9182113e2ad62ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354710
Auto-Submit: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797763}

ios/chrome/browser/ui/toolbar/toolbar_mediator.mm

@@ -321,6 +321,8 @@
 
 // Updates the Share Menu button of the consumer.
 - (void)updateShareMenuForWebState:(web::WebState*)webState {
+  if (!self.webState)
+    return;
   const GURL& URL = webState->GetLastCommittedURL();
   BOOL shareMenuEnabled =
       URL.is_valid() && !web::GetWebClient()->IsAppSpecificURL(URL);