[CSS Regions] Elements in a region should be assignable to a named flow

Source/core/rendering/FlowThreadController.cpp

Index: Source/core/rendering/FlowThreadController.cpp
diff --git a/Source/core/rendering/FlowThreadController.cpp b/Source/core/rendering/FlowThreadController.cpp
index 0970518497096535d2b5bfe9810a4e4a7a204ac4..d7a9f00e5bec0f5e45a433825eb448338a11db2a 100644
--- a/Source/core/rendering/FlowThreadController.cpp
+++ b/Source/core/rendering/FlowThreadController.cpp
@@ -226,6 +226,11 @@ void FlowThreadController::updateFlowThreadsIntoConstrainedPhase()
     }
 }
 
+bool FlowThreadController::isContentNodeRegisteredWithAnyNamedFlow(Node* contentNode) const
+{
+    return m_mapNamedFlowContentNodes.contains(contentNode);
+}
+
 #ifndef NDEBUG
 bool FlowThreadController::isAutoLogicalHeightRegionsCountConsistent() const
 {