[Device Service] Decouple NFC implementation from //content

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

+// Copyright 2017 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 android.app.Activity;
+
+import org.chromium.base.Callback;
+import org.chromium.base.annotations.CalledByNative;
+import org.chromium.device.nfc.NfcDelegate;
+
+/**
+ * A //content-specific implementation of the NfcDelegate interface. Maps NFC
+ * host ideas to their corresponding NfcHost objects, allowing the NFC
+ * implementation to access the Activity of the WebContents with which its
+ * requesting frame is associated.
+ */
+public class ContentNfcDelegate implements NfcDelegate {
+    @CalledByNative
+    static ContentNfcDelegate create() {
+        return new ContentNfcDelegate();
+    }
+
+    public void trackActivityForHost(int hostId, Callback<Activity> callback) {
+        NfcHost host = NfcHost.fromContextId(hostId);
+        if (host == null) {
+            return;
+        }
+        host.trackActivityChanges(callback);
+    }
+
+    public void stopTrackingActivityForHost(int hostId) {
+        NfcHost host = NfcHost.fromContextId(hostId);
+        if (host == null) {
+            return;
+        }
+        host.stopTrackingActivityChanges();
+    }
+}