Don't fire onbeforeunload for same document history navigations with PlzNavigate.

content/browser/frame_host/navigator_impl.cc

Index: content/browser/frame_host/navigator_impl.cc
diff --git a/content/browser/frame_host/navigator_impl.cc b/content/browser/frame_host/navigator_impl.cc
index 09f927e3793d8cba56b48111f6d02953393357b6..77eedb821c14ddf65d184d63feb67e9082089b16 100644
--- a/content/browser/frame_host/navigator_impl.cc
+++ b/content/browser/frame_host/navigator_impl.cc
@@ -1127,6 +1127,7 @@ void NavigatorImpl::RequestNavigation(FrameTreeNode* frame_tree_node,
   // This value must be set here because creating a NavigationRequest might
   // change the renderer live/non-live status and change this result.
   bool should_dispatch_beforeunload =
+      !is_same_document_history_load &&
       frame_tree_node->current_frame_host()->ShouldDispatchBeforeUnload();
   FrameMsg_Navigate_Type::Value navigation_type =
       GetNavigationType(controller_->GetBrowserContext(), entry, reload_type);