Introduce few const getters for dom classes

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

Index: third_party/WebKit/Source/core/dom/Document.h
diff --git a/third_party/WebKit/Source/core/dom/Document.h b/third_party/WebKit/Source/core/dom/Document.h
index 586f2396875d60c3d33bb1f71704ccacfe941c69..b01d5c18896946e8c913ba4de118252af4b7e506 100644
--- a/third_party/WebKit/Source/core/dom/Document.h
+++ b/third_party/WebKit/Source/core/dom/Document.h
@@ -276,6 +276,7 @@ public:
     DocumentType* doctype() const { return m_docType.get(); }
 
     DOMImplementation& implementation();
+    DOMImplementation* maybeImplementation() const;
 
     Element* documentElement() const
     {
@@ -383,6 +384,7 @@ public:
 
     // This is a DOM function.
     StyleSheetList* styleSheets();
+    StyleSheetList* maybeStyleSheets() const;
 
     StyleEngine& styleEngine() { DCHECK(m_styleEngine.get()); return *m_styleEngine.get(); }
 
@@ -536,6 +538,7 @@ public:
     void disableEval(const String& errorMessage) final;
 
     CSSStyleSheet& elementSheet();
+    CSSStyleSheet* maybeElementSheet() const { return m_elemSheet; }
 
     virtual DocumentParser* createParser();
     DocumentParser* parser() const { return m_parser.get(); }