(not part of build)

Add notion of CanHandleContent. This is supposed to be a light weight method for the implementor
such that the uri loader can ask right off the top if the listener can handle a particular content
type. If it can, then later on, the uir loader may call DoContent to actually handle it.

uriloader/base/nsIURIContentListener.idl

@@ -58,4 +58,8 @@ interface nsIURIContentListener : nsISupports
 				  in nsIChannel aOpenedChannel,
                   out nsIStreamListener aContentHandler,
 				  out boolean aAbortProcess);
+
+   /* returns true if we can handle the content and false if we can't */
+   boolean canHandleContent(in string aContentType, in string aCommand, 
+						 in string aWindowTarget);
 };