[iOS] Reset BVC._voiceSearchController to null in |-shutdown|.

The VoiceSearchController created by VoiceSearchProvider may depend on
state that is no longer valid after shut down.  Reset BVC's pointer to
it early so that shutdown can occur properly when the BVC is later
destroyed.

Bug: 1083208
Change-Id: I05dffce9eb79d353443eae759bc990fb61d74308
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216804
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772250}

ios/chrome/browser/ui/browser_view/browser_view_controller.mm

@@ -1321,8 +1321,10 @@ NSString* const kBrowserViewControllerSnackbarCategory =
   _sideSwipeController = nil;
   _webStateListObserver.reset();
   _allWebStateObservationForwarder = nullptr;
-  if (_voiceSearchController)
+  if (_voiceSearchController) {
     _voiceSearchController->SetDispatcher(nil);
+    _voiceSearchController = nullptr;
+  }
   [[NSNotificationCenter defaultCenter] removeObserver:self];
 }