Use weak members to cache StyleSheetContents.

third_party/WebKit/Source/core/dom/StyleEngine.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 144 matching lines @@
     void removeFontFaceRules(const HeapVector<Member<const StyleRuleFontFace>>&);
     void clearFontCache();
     // updateGenericFontFamilySettings is used from WebSettingsImpl.
     void updateGenericFontFamilySettings();
 
     void didDetach();
     bool shouldClearResolver() const;
     void resolverChanged(StyleResolverUpdateMode);
 
     CSSStyleSheet* createSheet(Element*, const String& text, TextPosition startPosition, StyleEngineContext&);
-    void removeSheet(StyleSheetContents*);
 
     void collectScopedStyleFeaturesTo(RuleFeatureSet&) const;
     void ensureFullscreenUAStyle();
 
     void platformColorsChanged();
 
     void classChangedForElement(const SpaceSplitString& changedClasses, Element&);
     void classChangedForElement(const SpaceSplitString& oldClasses, const SpaceSplitString& newClasses, Element&);
     void attributeChangedForElement(const QualifiedName& attributeName, Element&);
     void idChangedForElement(const AtomicString& oldId, const AtomicString& newId, Element&);
@@ skipping 84 matching lines @@
 
     bool m_ignorePendingStylesheets = false;
     bool m_didCalculateResolver = false;
     bool m_hasUnresolvedKeyframesRule = false;
 
     Member<StyleResolver> m_resolver;
     StyleInvalidator m_styleInvalidator;
 
     Member<CSSFontSelector> m_fontSelector;
 
-    HeapHashMap<AtomicString, Member<StyleSheetContents>> m_textToSheetCache;
-    HeapHashMap<Member<StyleSheetContents>, AtomicString> m_sheetToTextCache;
+    HeapHashMap<AtomicString, WeakMember<StyleSheetContents>> m_textToSheetCache;
+    HeapHashMap<WeakMember<StyleSheetContents>, AtomicString> m_sheetToTextCache;
 
     std::unique_ptr<StyleResolverStats> m_styleResolverStats;
     unsigned m_styleForElementCount = 0;
 
     friend class StyleEngineTest;
 };
 
 } // namespace blink
 
 #endif