iOS: Re-add bits of GTM we actually use.

ios/chrome/test/google_toolbox_unittest.mm

+// Copyright (c) 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.
+
+#import <Foundation/Foundation.h>
+
+#include "base/logging.h"
+#include "testing/gtest_mac.h"
+#import "third_party/google_toolbox_for_mac/src/Foundation/GTMNSDictionary+URLArguments.h"
+
+// [NSDictionary gtm_dictionaryWithHttpArgumentsString] is used downstream.
+// This test ensures that we keep compiling the file.
+TEST(GoogleToolboxForMacTest, dictionaryWithHttpArgumentsString) {
+  NSDictionary* dict = [NSDictionary gtm_dictionaryWithHttpArgumentsString:@""];
+  EXPECT_EQ(0u, [dict count]);
+}