Clear CSSGlobalRuleSet on StyleEngine::didDetach().

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

Index: third_party/WebKit/Source/core/css/CSSGlobalRuleSet.cpp
diff --git a/third_party/WebKit/Source/core/css/CSSGlobalRuleSet.cpp b/third_party/WebKit/Source/core/css/CSSGlobalRuleSet.cpp
index df122f876f9ab97781a4a6bd1b11ea2bd837fb08..97301d6521b7b1cc42b0d605eba1e191bea9b8c6 100644
--- a/third_party/WebKit/Source/core/css/CSSGlobalRuleSet.cpp
+++ b/third_party/WebKit/Source/core/css/CSSGlobalRuleSet.cpp
@@ -69,6 +69,15 @@ void CSSGlobalRuleSet::update(Document& document) {
   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);