Add a destroy method to the TabModelSelector interface.

chrome/android/java/src/org/chromium/chrome/browser/tabmodel/SingleTabModel.java

Index: chrome/android/java/src/org/chromium/chrome/browser/tabmodel/SingleTabModel.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/SingleTabModel.java b/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/SingleTabModel.java
index ed70f0bea232721009aa0a96eec45a705caa49f7..ff17f41e6622c973fbc55090623b55e3ba0cf308 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/SingleTabModel.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/SingleTabModel.java
@@ -73,7 +73,7 @@ public class SingleTabModel implements TabModel {
 
     @Override
     public int index() {
-        return 0;
+        return mTab != null ? 0 : INVALID_TAB_INDEX;
     }
 
     @Override
@@ -130,6 +130,12 @@ public class SingleTabModel implements TabModel {
     }
 
     @Override
+    public void destroy() {
+        if (mTab != null) mTab.destroy();
+        mTab = null;
+    }
+
+    @Override
     public Tab getNextTabIfClosed(int id) {
         return null;
     }
@@ -175,11 +181,6 @@ public class SingleTabModel implements TabModel {
         mObservers.removeObserver(observer);
     }
 
-    // Below are functions that are overridden but should be moved out of TabModel.
-    @Override
-    public void destroy() {
-    }
-
     private static native void nativePermanentlyBlockAllNewWindows(Tab nativeTabAndroid);
 
 }