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

content/public/android/java/src/org/chromium/content/browser/WindowAndroidProvider.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;
+
+/**
+ * WindowAndroidProvider is an interface that provides functionality to get WindowAndroid and
+ * observe changes whenver WindowAndroid is updated by the class that implements this interface.
+ */
+public interface WindowAndroidProvider {
+    /**
+     * Gets WindowAndroid.
+     */
+    WindowAndroid getWindowAndroid();
+
+    /**
+     * Adds WindowAndroidChangedObserver observer.
+     */
+    void addWindowAndroidChangedObserver(WindowAndroidChangedObserver observer);
+
+    /**
+     * Removes WindowAndroidChangedObserver observer.
+     */
+    void removeWindowAndroidChangedObserver(WindowAndroidChangedObserver observer);
+}