[webnfc] Implement push method for Android nfc mojo service.

content/public/android/java/src/org/chromium/content/browser/WindowAndroidChangedObserver.java

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.content.browser;
+
+import org.chromium.ui.base.WindowAndroid;
+
+/**
+ * Observer interface that is used by WindowAndroidProvider to notify observers when
+ * WindowAndroidProvider changes its window.
+ */
+public interface WindowAndroidChangedObserver {
+    /**
+     * Notifies observer when WindowAndroid is changed.
+     */
+    void onWindowAndroidChanged(WindowAndroid newWindowAndroid);
+}