Add postMessage APIs to the support lib

customtabs/src/android/support/customtabs/CustomTabsSession.java

Index: customtabs/src/android/support/customtabs/CustomTabsSession.java
diff --git a/customtabs/src/android/support/customtabs/CustomTabsSession.java b/customtabs/src/android/support/customtabs/CustomTabsSession.java
index 290150db1837a46d9a91c9aae52396cbafaf3cf7..fe02bcc7b3c14faaca9b32de622b4baa2a40fcdd 100644
--- a/customtabs/src/android/support/customtabs/CustomTabsSession.java
+++ b/customtabs/src/android/support/customtabs/CustomTabsSession.java
@@ -24,6 +24,7 @@ import android.os.Bundle;
 import android.os.IBinder;
 import android.os.RemoteException;
 import android.support.annotation.NonNull;
+import android.support.customtabs.CustomTabsService.Result;
 import android.support.customtabs.CustomTabsSessionToken.DummyCallback;
 import android.widget.RemoteViews;
 
@@ -150,6 +151,44 @@ public final class CustomTabsSession {
         }
     }
 
+    /**
+     * Sends a request to the implementation to validate and assign a postMessage origin for the
+     * related {@link CustomTabsSession}.
+     *
+     * <p>This also acts a trigger to setup a postMessage communication channel.
+     *
+     * @return Whether the implementation accepted the validation request. Note that returning true
+     *         here doesn't mean an origin has already been assigned as the validation can be
+     *         asynchronous.
+     */
+    public boolean validatePostMessageOrigin() {
+        try {
+            return mService.validatePostMessageOrigin(mCallback);
+        } catch (RemoteException e) {
+            return false;
+        }
+    }
+
+    /**
+     * Sends a postMessage request using the origin that has been validated and communicated via
+     * {@link CustomTabsCallback#onMessageChannelReady(Uri, Bundle)}. If postMessage() is called
+     * from a single thread, then the messages will be posted in the same order to the web page.
+     * Returns false when called without a preceding
+     * {@link CustomTabsService#validatePostMessageOrigin(CustomTabsSessionToken)}.
+     *
+     * @param message The message that is being sent.
+     * @param extras Reserved for future use.
+     * @return An integer constant about the postMessage request result.
+     */
+    @Result
+    public synchronized int postMessage(String message, Bundle extras) {
+        try {
+            return mService.postMessage(mCallback, message, extras);
+        } catch (RemoteException e) {
+            return CustomTabsService.RESULT_FAILURE_REMOTE_ERROR;
+        }
+    }
+
     /* package */ IBinder getBinder() {
         return mCallback.asBinder();
     }