Introduce authentication service.

This service allows to request oauth2 token. The only current
implementation is for Android.

Also the implementation is currently using a single project for
authentication.

R=ppi@chromium.org, tonyg@chromium.org
BUG=https://github.com/domokit/mojo/issues/94

Review URL: https://codereview.chromium.org/1116653002
diff --git a/services/android/intent_receiver.mojom b/services/android/intent_receiver.mojom
index a61df69..caa2f3f 100644
--- a/services/android/intent_receiver.mojom
+++ b/services/android/intent_receiver.mojom
@@ -12,13 +12,23 @@
   // caller can then transform this intent into a PendingIntent using
   // |PendingIntent#getService| and send it to another android application.
   // Whenever the pending intent is executed, the receiver will be called with
-  // the content of the received intent. To be noted: this will fail if the
-  // received intent is active (contains either a Binder or a file descriptor).
-  RegisterReceiver(IntentReceiver receiver) => (array<uint8> intent);
+  // the content of the received intent.
+  // To be noted, this will fail if the received intent is active (contains
+  // either a Binder or a file descriptor).
+  RegisterIntentReceiver(IntentReceiver receiver) => (array<uint8>? intent);
+
+  // This method takes an |IntentReceiver| and returns a serialized intent.
+  // The serialized intent can be deserialized using an android parcel. The
+  // caller can then add an intent it wants the system to send using
+  // |Activity#startActivityForResult| and then send it using
+  // |Context#startService|. Whenever the started activity sends a result, the
+  // receiver will be called with the content of the received intent. If the
+  // activity is cancelled, the receiver will be closed.
+  RegisterActivityResultReceiver(IntentReceiver receiver) =>
+      (array<uint8>? intent);
 };
 
-// Receiver interface, to be used with
-// |IntentReceiverManager.RegisterReceiver|.
+// Receiver interface, to be used with |IntentReceiverManager|.
 interface IntentReceiver {
   OnIntent(array<uint8> intent);
 };