Bug 1488684 - Made nsIFrame-derived classes and some others 'final' where possible - r=dbaron

All classes deriving from nsIFrame that did not have any subclasses themselves
(at the time of writing this patch) have been marked with `final`.
Some other Layout classes have also been made final, but this was opportunistic
while working on nsIFrame subclasses, and is definitely not exhaustive, further
patches welcome; refer to bug 1332680.

Advantages of marking a class final include:
- Allowing the compiler to devirtualize some method calls (i.e., calling
  virtual functions directly instead of going through the vtable),
- Indicating that the class is not currently subclassed,
- Preventing subclassing without being aware that this would remove the
  finalization benefits of the parent class.
`final` does not signify that these classes should *never* be subclassed, this
is left for developers to decide.

Differential Revision: https://phabricator.services.mozilla.com/D5020

Author: squelart

layout/base/nsDocumentViewer.cpp

@@ -148,7 +148,7 @@ class AutoPrintEventDispatcher;
 
 // a small delegate class used to avoid circular references
 
-class nsDocViewerSelectionListener : public nsISelectionListener
+class nsDocViewerSelectionListener final : public nsISelectionListener
 {
 public:
 
@@ -180,7 +180,7 @@ class nsDocViewerSelectionListener : public nsISelectionListener
 
 /** editor Implementation of the FocusListener interface
  */
-class nsDocViewerFocusListener : public nsIDOMEventListener
+class nsDocViewerFocusListener final : public nsIDOMEventListener
 {
 public:
   /** default constructor

layout/base/nsFrameTraversal.h

@@ -22,7 +22,7 @@ nsresult NS_NewFrameTraversal(nsIFrameEnumerator **aEnumerator,
 
 nsresult NS_CreateFrameTraversal(nsIFrameTraversal** aResult);
 
-class nsFrameTraversal : public nsIFrameTraversal
+class nsFrameTraversal final : public nsIFrameTraversal
 {
 public:
   nsFrameTraversal();