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

device/nfc/android/java/src/org/chromium/device/nfc/NfcMessageValidator.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.device.nfc;
+
+import org.chromium.device.nfc.mojom.NfcMessage;
+import org.chromium.device.nfc.mojom.NfcRecord;
+import org.chromium.device.nfc.mojom.NfcRecordType;
+
+/**
+ * Utility class that provides validation of NfcMessage.
+ */
+public final class NfcMessageValidator {
+    /**
+     * Validates NfcMessage.
+     *
+     * @param message to be validated.
+     * @return true if message is valid, false otherwise.
+     */
+    public static boolean isValid(NfcMessage message) {
+        if (message == null || message.data == null || message.data.length == 0) {
+            return false;
+        }
+
+        for (int i = 0; i < message.data.length; ++i) {
+            if (!isValid(message.data[i])) return false;
+        }
+        return true;
+    }
+
+    private static boolean isValid(NfcRecord record) {
+        if (record == null || record.data == null || record.data.length == 0) {
+            return false;
+        }
+
+        if (record.mediaType == null || record.mediaType.isEmpty()
+                || record.recordType == NfcRecordType.EMPTY) {
+            return false;
+        }
+
+        return true;
+    }
+}