Add postMessage APIs to the support lib

customtabs/src/android/support/customtabs/CustomTabsCallback.java

Index: customtabs/src/android/support/customtabs/CustomTabsCallback.java
diff --git a/customtabs/src/android/support/customtabs/CustomTabsCallback.java b/customtabs/src/android/support/customtabs/CustomTabsCallback.java
index d7fdd3988d23385b3009c85b0532ccb8418ecf4c..9b649d5ae57eed617f5977a05da06dc3998b4656 100644
--- a/customtabs/src/android/support/customtabs/CustomTabsCallback.java
+++ b/customtabs/src/android/support/customtabs/CustomTabsCallback.java
@@ -16,6 +16,7 @@
 
 package android.support.customtabs;
 
+import android.net.Uri;
 import android.os.Bundle;
 
 /**
@@ -62,6 +63,27 @@ public class CustomTabsCallback {
     public void onNavigationEvent(int navigationEvent, Bundle extras) {}
 
     /**
+     * Sent when {@link CustomTabsSession} has requested a postMessage channel through
+     * {@link CustomTabsService#validatePostMessageOrigin(CustomTabsSessionToken)} and the channel
+     * is ready for sending and receiving messages on both ends.
+     * @param origin The web origin that has been validated and will be used in all messages coming
+     *               from this {@link CustomTabsSession}.
+     * @param extras Reserved for future use.
+     */
+    public void onMessageChannelReady(Uri origin, Bundle extras) {}
+
+    /**
+     * Sent when a tab controlled by this {@link CustomTabsSession} has sent a postMessage message.
+     * This method is syncronized to keep the ordering of messages sent across the binder thread

[Benoit L, 2016/11/08 16:27:06]

nit: synchronized

[Yusuf, 2016/11/08 18:59:10]

Done.

+     * pool. When received on the client side, it is the client's responsibility to preserve the

[Benoit L, 2016/11/08 16:27:06]

nit: Similar to postMessage(), should we actually only document the ordering
guarantees?

[Yusuf, 2016/11/08 18:59:10]

Done.

+     * ordering further.
+     *
+     * @param message The message sent.
+     * @param extras Reserved for future use.
+     */
+    public synchronized void onPostMessage(String message, Bundle extras) {}
+
+    /**
      * Unsupported callbacks that may be provided by the implementation.
      *
      * <p>