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

util/net/http_headers.h

Index: util/net/http_headers.h
diff --git a/util/net/http_headers.h b/util/net/http_headers.h
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+// 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.
+
+#ifndef CRASHPAD_UTIL_NET_HTTP_HEADERS_H_
+#define CRASHPAD_UTIL_NET_HTTP_HEADERS_H_
+
+#include <map>
+#include <string>
+
+namespace crashpad {
+
+//! \brief A map of HTTP header fields to their values.
+using HTTPHeaders = std::map<std::string, std::string>;
+
+//! \brief The header name `"Content-Type"`.
+extern const char kContentType[];
+
+//! \brief The header name `"Content-Length"`.
+extern const char kContentLength[];
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_UTIL_NET_HTTP_HEADERS_H_