Move StyleSheetContents cache to StyleEngine.

Source/core/css/StyleSheetContents.h

Index: Source/core/css/StyleSheetContents.h
diff --git a/Source/core/css/StyleSheetContents.h b/Source/core/css/StyleSheetContents.h
index 8090f8e4b4df48c115f1d95bd2819a51c0266376..90acc818cd9c85ab313221d08a4defd2cfbeb9a1 100644
--- a/Source/core/css/StyleSheetContents.h
+++ b/Source/core/css/StyleSheetContents.h
@@ -139,13 +139,16 @@ public:
 
     void registerClient(CSSStyleSheet*);
     void unregisterClient(CSSStyleSheet*);
-    bool hasOneClient() { return (m_loadingClients.size() + m_completedClients.size()) == 1; }
+    size_t clientSize() const { return m_loadingClients.size() + m_completedClients.size(); }
+    bool hasOneClient() { return clientSize() == 1; }
     void clientLoadCompleted(CSSStyleSheet*);
     void clientLoadStarted(CSSStyleSheet*);
 
     bool isMutable() const { return m_isMutable; }
     void setMutable() { m_isMutable = true; }
 
+    void removeSheetFromCache(Document*);
+
     bool isInMemoryCache() const { return m_isInMemoryCache; }
     void addedToMemoryCache();
     void removedFromMemoryCache();
@@ -165,6 +168,7 @@ private:
     StyleSheetContents(const StyleSheetContents&);
     void notifyRemoveFontFaceRule(const StyleRuleFontFace*);
 
+    Document* clientSingleOwnerDocument() const;
     void clearCharsetRule();
 
     RawPtrWillBeMember<StyleRuleImport> m_ownerRule;
@@ -184,6 +188,7 @@ private:
     bool m_isInMemoryCache : 1;
     bool m_hasFontFaceRule : 1;
     bool m_hasMediaQueries : 1;
+    bool m_hasSingleOwnerDocument : 1;
 
     CSSParserContext m_parserContext;