Make a microtask of distribution recalc happen only when ShadowDOMV1 flag is enabled.

third_party/WebKit/Source/core/dom/Node.cpp

Index: third_party/WebKit/Source/core/dom/Node.cpp
diff --git a/third_party/WebKit/Source/core/dom/Node.cpp b/third_party/WebKit/Source/core/dom/Node.cpp
index f1caf2f053eeafdd9b5f50f2e675ecbc741f5613..7e548db9ab627b4d60c3146a073d22d9287f2f1b 100644
--- a/third_party/WebKit/Source/core/dom/Node.cpp
+++ b/third_party/WebKit/Source/core/dom/Node.cpp
@@ -86,6 +86,7 @@
 #include "core/page/Page.h"
 #include "core/svg/graphics/SVGImage.h"
 #include "platform/EventDispatchForbiddenScope.h"
+#include "platform/RuntimeEnabledFeatures.h"
 #include "platform/TraceEvent.h"
 #include "platform/TracedValue.h"
 #include "wtf/HashSet.h"
@@ -691,7 +692,7 @@ void Node::markAncestorsWithChildNeedsStyleInvalidation()
 void Node::markAncestorsWithChildNeedsDistributionRecalc()
 {
     ScriptForbiddenScope forbidScriptDuringRawIteration;
-    if (inDocument() && !document().childNeedsDistributionRecalc()) {
+    if (RuntimeEnabledFeatures::shadowDOMV1Enabled() && inDocument() && !document().childNeedsDistributionRecalc()) {
         // TODO(hayato): Support a non-document composed tree.
         // TODO(hayato): Enqueue a task only if a 'slotchange' event listner is registered in the document composed tree.
         Microtask::enqueueMicrotask(WTF::bind(&Document::updateDistribution, PassRefPtrWillBeRawPtr<Document>(&document())));