Adds UMA for slow and fast back/forward WKWebController navigations.

ios/web/web_state/ui/crw_wk_web_view_web_controller.mm

Index: ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm b/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
index 5cedc12de5b5f2fe4733f156fbe920bd7f2dc233..4663b1fc546bb59107b66e81b5adf8f3ae3765aa 100644
--- a/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
+++ b/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
@@ -741,6 +741,14 @@ WKWebViewErrorSource WKWebViewErrorSourceFromError(NSError* error) {
   [super loadCancelled];
 }
 
+- (BOOL)canPerformFastNavigationForSessionEntry:(CRWSessionEntry*)sessionEntry {
+  web::WKBackForwardListItemHolder* holder =
+      web::WKBackForwardListItemHolder::FromNavigationItem(
+          sessionEntry.navigationItemImpl);
+  return (holder->back_forward_list_item() &&
+          [self isBackForwardListItemValid:holder->back_forward_list_item()]);
+}
+
 - (BOOL)isViewAlive {
   return !_webProcessIsDead && [super isViewAlive];
 }