Fix for BlackScreen Bug

Cancel request to go to the NavScreen
and discard callback if tab state changes.

Change-Id: I3b29023c10c2a3821f36a43e465df27d663c0825
diff --git a/src/com/android/browser/UI.java b/src/com/android/browser/UI.java
index e916a35..172509b 100644
--- a/src/com/android/browser/UI.java
+++ b/src/com/android/browser/UI.java
@@ -60,6 +60,8 @@
 
     public void setActiveTab(Tab tab);
 
+    public void cancelNavScreenRequest();
+
     public void updateTabs(List<Tab> tabs);
 
     public void detachTab(Tab tab);