(Set)IsOverridingUserAgent should be called on VisibleItem

ios/web/public/navigation_manager.h

Index: ios/web/public/navigation_manager.h
diff --git a/ios/web/public/navigation_manager.h b/ios/web/public/navigation_manager.h
index bec6c696e9d91976aa8beacb7be49e8835d33c52..ec592aaae51aff17a90b4b618123d0ae2afb35e8 100644
--- a/ios/web/public/navigation_manager.h
+++ b/ios/web/public/navigation_manager.h
@@ -154,6 +154,10 @@ class NavigationManager {
   // TODO(crbug.com/692303): Remove this when overriding the user agent doesn't
   // create a new NavigationItem.
   virtual void OverrideDesktopUserAgentForNextPendingItem() = 0;
+
+  // Whether or not desktop user agent is used for the currently loaded page.
+  // Returns true, if use a desktop user agent and false for a mobile one.
+  virtual bool IsUsingDesktopUserAgent() const = 0;
 };
 
 }  // namespace web