Support slot element's fallback content feature

third_party/WebKit/Source/core/html/HTMLSlotElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
index cf907c221c755daa2c33147e415d07648ad7d523..a3f1e37a55b40d6af3eecd960ff06567795d1f07 100644
--- a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
@@ -31,6 +31,8 @@
 #include "core/html/HTMLSlotElement.h"
 
 #include "core/HTMLNames.h"
+#include "core/dom/NodeTraversal.h"
+#include "core/dom/shadow/InsertionPoint.h"
 
 namespace blink {
 
@@ -98,6 +100,21 @@ void HTMLSlotElement::detach(const AttachContext& context)
     HTMLElement::detach(context);
 }
 
+void HTMLSlotElement::updateDistributedNodesWithFallback()
+{
+    if (!m_distributedNodes.isEmpty())
+        return;
+    for (auto& child : NodeTraversal::childrenOf(*this)) {
+        // Insertion points are not supported as slots fallback
+        if (isActiveInsertionPoint(child))
+            continue;
+        if (isHTMLSlotElement(child))
+            appendDistributedNodes(toHTMLSlotElement(child).getDistributedNodes());
+        else
+            appendDistributedNode(child);
+    }
+}
+
 DEFINE_TRACE(HTMLSlotElement)
 {
     visitor->trace(m_assignedNodes);