Add the Mojo implementation of authenticator.mojom's MakeCredential.

components/webauth/authenticator_web_contents_manager.cc

Index: components/webauth/authenticator_web_contents_manager.cc
diff --git a/components/webauth/authenticator_web_contents_manager.cc b/components/webauth/authenticator_web_contents_manager.cc
new file mode 100644
index 0000000000000000000000000000000000000000..a976b2ade40fc37d63c172d0688bda925203fae6
--- /dev/null
+++ b/components/webauth/authenticator_web_contents_manager.cc
@@ -0,0 +1,38 @@
+// 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.
+
+#include "components/webauth/authenticator_web_contents_manager.h"
+
+#include <utility>
+
+#include "base/logging.h"
+#include "base/memory/ptr_util.h"
+#include "components/webauth/authenticator_impl.h"
+#include "content/public/browser/web_contents.h"
+
+DEFINE_WEB_CONTENTS_USER_DATA_KEY(AuthenticatorWebContentsManager);
+
+AuthenticatorWebContentsManager::~AuthenticatorWebContentsManager() {}
+
+AuthenticatorWebContentsManager*
+AuthenticatorWebContentsManager::GetOrCreateForWebContents(
+    content::WebContents* web_contents) {
+  DCHECK(web_contents);
+  // Does nothing if the manager instance already exists.
+  AuthenticatorWebContentsManager* manager = FromWebContents(web_contents);
+  if (!manager) {
+    CreateForWebContents(web_contents);
+    manager = FromWebContents(web_contents);
+  }
+  return manager;
+}
+
+void AuthenticatorWebContentsManager::CreateAuthenticator(
+    content::RenderFrameHost* render_frame_host,
+    mojo::InterfaceRequest<webauth::mojom::Authenticator> request) {
+  webauth::AuthenticatorImpl::Create(render_frame_host, std::move(request));
+}
+
+AuthenticatorWebContentsManager::AuthenticatorWebContentsManager(
+    content::WebContents* web_contents) {}