FacebookApi: Use access token json extractor

Fixes: [1] / #103.

[1] https://github.com/davido/gerrit-oauth-provider/issues/103

Change-Id: I14ea93a857a2f9cfb425ede1f7923ec42f221931
diff --git a/src/main/java/com/googlesource/gerrit/plugins/oauth/Facebook2Api.java b/src/main/java/com/googlesource/gerrit/plugins/oauth/Facebook2Api.java
new file mode 100644
index 0000000..a547bfb
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/oauth/Facebook2Api.java
@@ -0,0 +1,25 @@
+// Copyright (C) 2018 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.googlesource.gerrit.plugins.oauth;
+
+import org.scribe.builder.api.FacebookApi;
+import org.scribe.extractors.AccessTokenExtractor;
+
+public class Facebook2Api extends FacebookApi {
+  @Override
+  public AccessTokenExtractor getAccessTokenExtractor() {
+    return OAuth2AccessTokenJsonExtractor.instance();
+  }
+}
diff --git a/src/main/java/com/googlesource/gerrit/plugins/oauth/FacebookOAuthService.java b/src/main/java/com/googlesource/gerrit/plugins/oauth/FacebookOAuthService.java
index 44d4b36..d3ebe8e 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/oauth/FacebookOAuthService.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/oauth/FacebookOAuthService.java
@@ -32,7 +32,6 @@
 import java.io.IOException;
 import javax.servlet.http.HttpServletResponse;
 import org.scribe.builder.ServiceBuilder;
-import org.scribe.builder.api.FacebookApi;
 import org.scribe.model.OAuthRequest;
 import org.scribe.model.Response;
 import org.scribe.model.Token;
@@ -65,7 +64,7 @@
 
     service =
         new ServiceBuilder()
-            .provider(FacebookApi.class)
+            .provider(Facebook2Api.class)
             .apiKey(cfg.getString(InitOAuth.CLIENT_ID))
             .apiSecret(cfg.getString(InitOAuth.CLIENT_SECRET))
             .callback(canonicalWebUrl + "oauth")