Bug 1386411 - Part 2: Devirtualize and inline nsISelection::AsSelection(); r=bzbarsky

nsISelection is builtinclass, so this method doesn't need to be virtual.

dom/base/Selection.h

@@ -73,8 +73,6 @@ public:
   NS_DECL_NSISELECTION
   NS_DECL_NSISELECTIONPRIVATE
 
-  virtual Selection* AsSelection() override { return this; }
-
   nsresult EndBatchChangesInternal(int16_t aReason = nsISelectionListener::NO_REASON);
 
   nsIDocument* GetParentObject() const;
@@ -519,4 +517,10 @@ public:
 } // namespace dom
 } // namespace mozilla
 
+inline mozilla::dom::Selection*
+nsISelection::AsSelection()
+{
+  return static_cast<mozilla::dom::Selection*>(this);
+}
+
 #endif // mozilla_Selection_h__

dom/base/nsISelection.idl

@@ -161,10 +161,11 @@ interface nsISelection : nsISupports
 
 %{C++
     /**
-     * AsSelection() returns a pointer to Selection class if the instance is
-     * derived from it.  Otherwise, nullptr but it should never happen
-     * since Selection is the only class implementing nsISelection.
+     * AsSelection() returns a pointer to Selection class.
+     *
+     * In order to avoid circular dependency issues, this method is defined
+     * in mozilla/dom/Selection.h.  Consumers need to #include that header.
      */
-    virtual mozilla::dom::Selection* AsSelection() = 0;
+    inline mozilla::dom::Selection* AsSelection();
 %}
 };