create promo for debug.

third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp

Index: third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
index 2f576663e6ef833eda4cea6fa42d4172fb10743f..980b1e6d046093863ae2cd91e390fae1aac0e318 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
@@ -1208,6 +1208,14 @@ StyleRuleKeyframes* StyleResolver::FindKeyframesRule(
           element->GetTreeScope().GetScopedStyleResolver())
     resolvers.push_back(scoped_resolver);
 
+  if (element->IsInShadowTree() &&
+      ToShadowRoot(element->GetTreeScope().RootNode()).GetType() ==
+          ShadowRootType::kUserAgent) {
+    ScopedStyleResolver* scoped_resolver = element->GetDocument().GetScopedStyleResolver();
+    scoped_resolver->AddKeyframeRules(*CSSDefaultStyleSheets::Instance().DefaultStyle());
+    resolvers.push_back(scoped_resolver);
+  }
+
   for (auto& resolver : resolvers) {
     if (StyleRuleKeyframes* keyframes_rule =
             resolver->KeyframeStylesForAnimation(animation_name.Impl()))