6636331: ConcurrentModificationException in AppContext code

Summary: Added synchronization to AppContext.getAppContexts()
Reviewed-by: art

src/share/classes/sun/awt/AppContext.java

@@ -146,8 +146,10 @@ public final class AppContext {
      * Returns a set containing all <code>AppContext</code>s.
      */
     public static Set<AppContext> getAppContexts() {
+        synchronized (threadGroup2appContext) {
             return new HashSet<AppContext>(threadGroup2appContext.values());
         }
+    }
 
     /* The main "system" AppContext, used by everything not otherwise
        contained in another AppContext.