Reland of (Set)IsOverridingUserAgent should be called on VisibleItem

ios/chrome/browser/tabs/tab.mm

Index: ios/chrome/browser/tabs/tab.mm
diff --git a/ios/chrome/browser/tabs/tab.mm b/ios/chrome/browser/tabs/tab.mm
index e8e4d245ae79eb7ce67ed26d4ce2e50fc2850ba7..f669f1bfbc80ababda2f0e9c34eae5e1e1b9fb05 100644
--- a/ios/chrome/browser/tabs/tab.mm
+++ b/ios/chrome/browser/tabs/tab.mm
@@ -1550,13 +1550,16 @@ - (void)openAppStore:(NSString*)appId {
   [storeKitLauncher_ openAppStore:appId];
 }
 
-- (BOOL)useDesktopUserAgent {
-  web::NavigationItem* currentItem = self.currentNavigationItem;
-  return currentItem && currentItem->IsOverridingUserAgent();
+- (BOOL)usesDesktopUserAgent {
+  if (!self.navigationManager)
+    return NO;
+
+  web::NavigationItem* visibleItem = self.navigationManager->GetVisibleItem();
+  return visibleItem && visibleItem->IsOverridingUserAgent();
 }
 
 - (void)enableDesktopUserAgent {
-  DCHECK_EQ(self.useDesktopUserAgent, NO);
+  DCHECK_EQ(self.usesDesktopUserAgent, NO);
   DCHECK([self navigationManager]);
   [self navigationManager]->OverrideDesktopUserAgentForNextPendingItem();
 }