[Android] Extract a TabModelSelectorObserver to make it more consistent.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelector.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelector.java b/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelector.java
index c8f732e68b0372a12d3ed340070e6861f24e699b..20db582f7c66412d4d9f2da897702531a3c75df4 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelector.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelector.java
@@ -20,6 +20,7 @@ public interface TabModelSelector {
     /**
      * Interface of listener that get notified on changes in the {@link TabModel}s.
      */
+    // TODO(tedchoc): Remove this once all callers have switched to TabModelSelectorObserver.
     public interface ChangeListener {
         /**
          * Called whenever the {@link TabModel} has changed.
@@ -138,15 +139,29 @@ public interface TabModelSelector {
      * can be registered at the same time.
      * @param changeListener The {@link TabModelSelector.ChangeListener} to notify.
      */
+    // TODO(tedchoc): Remove this once all callers switch to addObserver.
     void registerChangeListener(ChangeListener changeListener);
 
     /**
      * Unregisters the listener.
      * @param changeListener The {@link TabModelSelector.ChangeListener} to remove.
      */
+    // TODO(tedchoc): Remove this once all callers switch to removeObserver.
     void unregisterChangeListener(ChangeListener changeListener);
 
     /**
+     * Add an observer to be notified of changes to the TabModelSelector.
+     * @param observer The {@link TabModelSelectorObserver} to notify.
+     */
+    void addObserver(TabModelSelectorObserver observer);
+
+    /**
+     * Removes an observer of TabModelSelector changes..
+     * @param observer The {@link TabModelSelectorObserver} to remove.
+     */
+    void removeObserver(TabModelSelectorObserver observer);
+
+    /**
      * Calls {@link TabModel#commitAllTabClosures()} on all {@link TabModel}s owned by this
      * selector.
      */