Add HTTPTransport, a Mac implementation, and an end-to-end test.

util/net/http_headers.cc

+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// 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.
+
+#include "util/net/http_headers.h"
+
+namespace crashpad {
+
+const char kContentType[] = "Content-Type";
+
+const char kContentLength[] = "Content-Length";
+
+}  // namespace crashpad