Split up ios/web:test_support.

ios/web/public/test/response_providers/string_response_provider.mm

-// Copyright 2015 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 "ios/web/public/test/response_providers/string_response_provider.h"
-
-namespace web {
-
-StringResponseProvider::StringResponseProvider(const std::string& response_body)
-    : response_body_(response_body) {}
-
-bool StringResponseProvider::CanHandleRequest(const Request& request) {
-  return true;
-}
-
-void StringResponseProvider::GetResponseHeadersAndBody(
-    const Request& request,
-    scoped_refptr<net::HttpResponseHeaders>* headers,
-    std::string* response_body) {
-  *headers = GetDefaultResponseHeaders();
-  *response_body = response_body_;
-}
-
-}  // namespace web