msvc barfs on functions taking nsAString& with default initializers from

an NS_LITERAL_STRING.  Remove the default initializer, which is not
being used.  r=jag, a=leaf to get tests building again.

xpcom/tests/TestObserverService.cpp

@@ -69,7 +69,7 @@ extern ostream &operator<<( ostream &s, nsString &str ) {
 
 class TestObserver : public nsIObserver, public nsSupportsWeakReference {
 public:
-    TestObserver( const nsAString &name = NS_LITERAL_STRING("unknown") )
+    TestObserver( const nsAString &name )
         : mName( name ) {
         NS_INIT_REFCNT();
     }