Issue #2721 - Create special case exception for <A>.Click() outside of DOM

This removes the requirement for there to be a non-null PresShell to
dispatch `Click()` events on `<A>` elements (only), since the exception
to the rule has propagated to the spec.

With these changes it should now be possible do create an anchor and
`Click()` on it from JS without actually first attaching it to the DOM
of the presented document, as abused by scripted downloads in pages
(instead of using the A attribute to custom-name downloads).

layout/base/nsPresContext.h

@@ -457,13 +457,9 @@ public:
 
   nsIDocShell* GetDocShell() const;
 
-  // XXX this are going to be replaced with set/get container
-  void SetLinkHandler(nsILinkHandler* aHandler) { mLinkHandler = aHandler; }
-  nsILinkHandler* GetLinkHandler() { return mLinkHandler; }
-
   /**
    * Detach this pres context - i.e. cancel relevant timers,
-   * SetLinkHandler(null), SetContainer(null) etc.
+   * SetContainer(null) etc.
    * Only to be used by the DocumentViewer.
    */
   virtual void Detach();
@@ -1245,10 +1241,6 @@ protected:
   nsIAtom* MOZ_UNSAFE_REF("always a static atom") mMedium; // initialized by subclass ctors
   nsCOMPtr<nsIAtom> mMediaEmulated;
 
-  // This pointer is nulled out through SetLinkHandler() in the destructors of
-  // the classes which set it. (using SetLinkHandler() again).
-  nsILinkHandler* MOZ_NON_OWNING_REF mLinkHandler;
-
   // Formerly mLangGroup; moving from charset-oriented langGroup to
   // maintaining actual language settings everywhere (see bug 524107).
   // This may in fact hold a langGroup such as x-western rather than