[iOS] Defend against nil WebState instead of DCHECKing.

If a Tab is leaked, it's possible for BVC's URLLoadingObserver callbacks
to be triggered after the Tab's corresponding WebState has been
destroyed.  This CL updates that implementation to prevent crashes if
this occurs.

Bug: 925757
Change-Id: Id47321d2555605e1c1634080c7601b4d0f0b969d
Reviewed-on: https://chromium-review.googlesource.com/c/1448686
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628449}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -3395,8 +3395,8 @@ NSString* const kBrowserViewControllerSnackbarCategory =
 
   WebStateList* webStateList = self.tabModel.webStateList;
   web::WebState* current_web_state = webStateList->GetActiveWebState();
-  DCHECK(current_web_state);
-  if (transitionType & ui::PAGE_TRANSITION_FROM_ADDRESS_BAR) {
+  if (current_web_state &&
+      (transitionType & ui::PAGE_TRANSITION_FROM_ADDRESS_BAR)) {
     bool isExpectingVoiceSearch =
         VoiceSearchNavigationTabHelper::FromWebState(current_web_state)
             ->IsExpectingVoiceSearch();