[Android] Add tab model selected observer method to the selector.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelectorObserver.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelectorObserver.java b/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelectorObserver.java
index beb58555a4df19dcce59861034dde71109aabd51..b0d20fa35d89a1cdbd244aa6d81157e250b9788e 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelectorObserver.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelectorObserver.java
@@ -19,4 +19,11 @@ public interface TabModelSelectorObserver {
      * Called when a new tab is created.
      */
     void onNewTabCreated(Tab tab);
+
+    /**
+     * Called when a different tab model has been selected.
+     * @param newModel The newly selected tab model.
+     * @param oldModel The previously selected tab model.
+     */
+    void onTabModelSelected(TabModel newModel, TabModel oldModel);
 }