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/BaseUi.java b/src/com/android/browser/BaseUi.java
index 4d8c5a7..b0f9129 100644
--- a/src/com/android/browser/BaseUi.java
+++ b/src/com/android/browser/BaseUi.java
@@ -281,7 +281,9 @@
     public void addTab(Tab tab) {
     }
 
-    @Override
+    public void cancelNavScreenRequest(){
+    }
+
     public void setActiveTab(final Tab tab) {
         if (tab == null) return;
         Tab tabToRemove = null;
@@ -298,6 +300,7 @@
             }
         }
         mActiveTab = tab;
+
         BrowserWebView web = (BrowserWebView) mActiveTab.getWebView();
         updateUrlBarAutoShowManagerTarget();
         attachTabToContentView(tab);
@@ -764,6 +767,7 @@
     }
 
     protected WebView getWebView() {
+
         if (mActiveTab != null) {
             return mActiveTab.getWebView();
         } else {