Clear CSSGlobalRuleSet on StyleEngine::didDetach().

third_party/WebKit/Source/core/css/CSSGlobalRuleSet.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/css/CSSGlobalRuleSet.h"
 
 #include "core/css/CSSDefaultStyleSheets.h"
 #include "core/css/RuleSet.h"
 #include "core/dom/CSSSelectorWatch.h"
 #include "core/dom/Document.h"
@@ skipping 51 matching lines @@
 
   if (m_watchedSelectorsRuleSet)
     m_features.add(m_watchedSelectorsRuleSet->features());
 
   document.styleEngine().collectScopedStyleFeaturesTo(m_features);
 
   m_siblingRuleSet = makeRuleSet(m_features.siblingRules());
   m_uncommonAttributeRuleSet = makeRuleSet(m_features.uncommonAttributeRules());
 }
 
+void CSSGlobalRuleSet::reset() {

[sof, 2016/12/20 18:42:09]

nit: is this leaving the object in a usable state? (..and will it even be used
again by the caller in Q here?) If not, then naming it dispose() would make its
purpose clearer and consistent with how we name these eager-release-of-resources
methods.

+  m_features.clear();
+  m_siblingRuleSet = nullptr;
+  m_uncommonAttributeRuleSet = nullptr;
+  m_watchedSelectorsRuleSet = nullptr;
+  m_hasFullscreenUAStyle = false;
+  m_isDirty = true;
+}
+
 DEFINE_TRACE(CSSGlobalRuleSet) {
   visitor->trace(m_features);
   visitor->trace(m_siblingRuleSet);
   visitor->trace(m_uncommonAttributeRuleSet);
   visitor->trace(m_watchedSelectorsRuleSet);
 }
 
 }  // namespace blink