Bug 762717: Tab's onDestroy needs to clear the HashMap, and can be executed in background. [r=margaret]

--HG--
extra : rebase_source : a9494cbbca15a30993d39f939df250d25b831db0

mobile/android/base/Tab.java

@@ -106,7 +106,7 @@ public final class Tab {
     }
 
     public void onDestroy() {
-        mDoorHangers = new HashMap<String, DoorHanger>();
+        mDoorHangers.clear();
         BrowserDB.unregisterContentObserver(mContentResolver, mContentObserver);
     }
 

mobile/android/base/Tabs.java

@@ -161,11 +161,11 @@ public class Tabs implements GeckoEventListener {
 
         int tabId = tab.getId();
         removeTab(tabId);
+        tab.onDestroy();
 
         GeckoApp.mAppContext.mMainHandler.post(new Runnable() { 
             public void run() {
                 notifyListeners(tab, TabEvents.CLOSED);
-                tab.onDestroy();
             }
         });