[ios] Only update tab for NTP when BVC is active.

Bug: 826369
Change-Id: I5bf700ff19089ded0c276139282fceb9f8de72e9
Reviewed-on: https://chromium-review.googlesource.com/c/1297259
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602190}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -5502,7 +5502,7 @@ nativeContentHeaderHeightForPreloadController:(PreloadController*)controller
     DCHECK(_ntpCoordinatorsForWebStates[webState]);
     _ntpCoordinatorsForWebStates.erase(webState);
   }
-  if (currentTab.webState == webState) {
+  if (self.active && currentTab.webState == webState) {
     [self displayTab:currentTab];
   }
 }