Invalidate style from shadow host, not root, to apply :host rules.

Source/core/css/invalidation/StyleSheetInvalidationAnalysis.cpp

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 159 matching lines @@
     if (classScopes.isEmpty() || !element->hasClass())
         return false;
     const SpaceSplitString& classNames = element->classNames();
     for (unsigned i = 0; i < classNames.size(); ++i) {
         if (classScopes.contains(classNames[i].impl()))
             return true;
     }
     return false;
 }
 
-static ContainerNode* outermostShadowHost(const ShadowRoot& root)
+static ContainerNode* outermostShadowHost(ContainerNode& host)
 {
-    ContainerNode* host = root.host();
-    while (host->isInShadowTree())
-        host = host->containingShadowRoot()->host();
-    return host;
+    ContainerNode* outerHost = &host;
+    while (outerHost->isInShadowTree())
+        outerHost = outerHost->containingShadowRoot()->host();
+    return outerHost;
 }
 
 void StyleSheetInvalidationAnalysis::invalidateStyle()
 {
     ASSERT(!m_dirtiesAllStyle);
 
     if (m_treeScope->rootNode().isShadowRoot()) {
-        ContainerNode* invalidationRoot = &m_treeScope->rootNode();
+        ContainerNode* invalidationRoot = toShadowRoot(m_treeScope->rootNode()).host();
         if (m_hasDistributedRules)
-            invalidationRoot = outermostShadowHost(*toShadowRoot(invalidationRoot));
+            invalidationRoot = outermostShadowHost(*invalidationRoot);
         invalidationRoot->setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::StyleSheetChange));
         return;
     }
 
     if (m_idScopes.isEmpty() && m_classScopes.isEmpty())
         return;
     Element* element = ElementTraversal::firstWithin(m_treeScope->document());
     while (element) {
         if (elementMatchesSelectorScopes(element, m_idScopes, m_classScopes)) {
             element->setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::StyleSheetChange));
             // The whole subtree is now invalidated, we can skip to the next sibling.
             element = ElementTraversal::nextSkippingChildren(*element);
             continue;
         }
         element = ElementTraversal::next(*element);
     }
 }
 
 }