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 57e12cdf66c11d1110710dc922d8aee4b3414593..6fac6d5708706305d36ad804451a4f4140b604b5 100644
--- a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
@@ -32,6 +32,8 @@
 #include "core/html/HTMLSlotElement.h"
 
 #include "core/HTMLNames.h"
+#include "core/dom/NodeTraversal.h"
+#include "core/dom/shadow/InsertionPoint.h"
 
 namespace blink {
 
@@ -99,6 +101,21 @@ 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);