(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 a6eda18221461e7f4268aae932b9dd11ecab1fdc..0d69467328dc7ac5bd2fbe16d9d7a285dba85adf 100644
--- a/ios/chrome/browser/tabs/tab.mm
+++ b/ios/chrome/browser/tabs/tab.mm
@@ -1567,13 +1567,13 @@ - (void)openAppStore:(NSString*)appId {
   [storeKitLauncher_ openAppStore:appId];
 }
 
-- (BOOL)useDesktopUserAgent {
-  web::NavigationItem* currentItem = self.currentNavigationItem;
-  return currentItem && currentItem->IsOverridingUserAgent();
+- (BOOL)isUsingDesktopUserAgent {
+  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();
 }