Break crashpad_handler into lib and exe for Windows

handler/handler_main.h

+// Copyright 2015 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_HANDLER_HANDLER_MAIN_H_
+#define CRASHPAD_HANDLER_HANDLER_MAIN_H_
+
+namespace crashpad {
+
+//! \brief The `main()` of the `crashpad_handler` binary.
+//!
+//! This is exposed so that `crashpad_handler` can be embedded into another
+//! binary, but called and used as if it were a standalone executable.
+int HandlerMain(int argc, char* argv[]);
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_HANDLER_HANDLER_MAIN_H_