[ios] Adds a TestSigninResourceProvider to TestChromeBrowserProvider.

ios/public/provider/chrome/browser/signin/test_signin_resources_provider.mm

+// 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.
+
+#include "ios/public/provider/chrome/browser/signin/test_signin_resources_provider.h"
+
+#include <UIKit/UIKit.h>
+
+TestSigninResourcesProvider::TestSigninResourcesProvider()
+    : default_avatar_([[UIImage alloc] init]) {}
+
+TestSigninResourcesProvider::~TestSigninResourcesProvider() {}
+
+UIImage* TestSigninResourcesProvider::GetDefaultAvatar() {
+  return default_avatar_;
+}
+
+NSString* TestSigninResourcesProvider::GetLocalizedString(
+    ios::SigninStringID string_id) {
+  return @"Test";
+}