Make Event.path() return a pre-calculated NodeList rather than calculating it dynamically.

Source/core/dom/EventRetargeter.cpp

Index: Source/core/dom/EventRetargeter.cpp
diff --git a/Source/core/dom/EventRetargeter.cpp b/Source/core/dom/EventRetargeter.cpp
index 95ab788e9177ead2a3b9da1dcb4bb8fc72a1d6d9..502a57092e0ba3773eb76bd6286eacb4e371bf67 100644
--- a/Source/core/dom/EventRetargeter.cpp
+++ b/Source/core/dom/EventRetargeter.cpp
@@ -20,6 +20,7 @@
 #include "config.h"
 #include "core/dom/EventRetargeter.h"
 
+#include "RuntimeEnabledFeatures.h"
 #include "core/dom/ContainerNode.h"
 #include "core/dom/EventContext.h"
 #include "core/dom/EventPathWalker.h"
@@ -103,6 +104,28 @@ void EventRetargeter::calculateEventPath(Node* node, Event* event)
             targetStack.removeLast();
         }
     }
+
+    // Calculates eventPath for each node for Event.path() API.
+    if (!RuntimeEnabledFeatures::experimentalShadowDOMEnabled())
+        return;
+    TreeScope* lastScope = 0;
+    size_t eventPathSize = eventPath.size();
+    for (size_t i = 0; i < eventPathSize; ++i) {
+        TreeScope* currentScope = eventPath[i]->node()->treeScope();
+        if (currentScope == lastScope) {
+            // Fast path.
+            eventPath[i]->setEventPath(eventPath[i - 1]->eventPath());
+            continue;
+        }
+        lastScope = currentScope;
+        Vector<RefPtr<Node> > nodes;
+        for (size_t j = 0; j < eventPathSize; ++j) {
+            Node* node = eventPath[j]->node();
+            if (node->treeScope()->isInclusiveAncestorOf(currentScope))
+                nodes.append(node);
+        }
+        eventPath[i]->adoptEventPath(nodes);
+    }
 }
 
 void EventRetargeter::adjustForMouseEvent(Node* node, MouseEvent& mouseEvent)