Enable posting a message from JS to Android webview.

android_webview/java/src/org/chromium/android_webview/MessagePort.java

+// Copyright 2015 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.android_webview;
+
+import android.util.Log;
+
+/**
+ * Represents the MessageChannel MessagePort object. Inspired from
+ * http://www.whatwg.org/specs/web-apps/current-work/multipage/web-messaging.html#message-channels
+ */
+public class MessagePort {
+
+    /**
+     * The interface for message handler for receiving messages. Called on a background thread.
+     */
+    public static interface MessageHandler {
+        void onMessage(String message);
+    };
+
+    private static final String TAG = "AwMessagePortService";
+
+    private int mPortId;
+    private MessageHandler mHandler;
+
+    public MessagePort(int portId) {
+        mPortId = portId;
+    }
+
+    public int portId() {
+        return mPortId;
+    }
+
+    public void setMessageHandler(MessageHandler handler) {
+        mHandler = handler;
+    }
+
+    public void onMessage(String message) {
+        if (mHandler == null) {
+            Log.w(TAG, "No handler set for port [" + mPortId + "], dropping message " + message);
+            return;
+        }
+        mHandler.onMessage(message);
+    }
+}