Bug 1551040 - Add const versions of Document casts. r=edgar

Depends on D101516

Differential Revision: https://phabricator.services.mozilla.com/D101517

dom/base/Document.h

@@ -3556,12 +3556,14 @@ class Document : public nsINode,
    * Defined inline in nsHTMLDocument.h
    */
   inline nsHTMLDocument* AsHTMLDocument();
+  inline const nsHTMLDocument* AsHTMLDocument() const;
 
   /**
    * Asserts IsSVGDocument, and can't return null.
    * Defined inline in SVGDocument.h
    */
   inline SVGDocument* AsSVGDocument();
+  inline const SVGDocument* AsSVGDocument() const;
 
   /*
    * Given a node, get a weak reference to it and append that reference to

dom/html/nsHTMLDocument.h

@@ -202,6 +202,11 @@ inline nsHTMLDocument* Document::AsHTMLDocument() {
   return static_cast<nsHTMLDocument*>(this);
 }
 
+inline const nsHTMLDocument* Document::AsHTMLDocument() const {
+  MOZ_ASSERT(IsHTMLOrXHTML());
+  return static_cast<const nsHTMLDocument*>(this);
+}
+
 }  // namespace dom
 }  // namespace mozilla
 

dom/svg/SVGDocument.h

@@ -43,6 +43,11 @@ inline SVGDocument* Document::AsSVGDocument() {
   return static_cast<SVGDocument*>(this);
 }
 
+inline const SVGDocument* Document::AsSVGDocument() const {
+  MOZ_ASSERT(IsSVGDocument());
+  return static_cast<const SVGDocument*>(this);
+}
+
 }  // namespace dom
 }  // namespace mozilla