Add ConstThreadState to mach_extensions.h and use it everywhere

handler/mac/crash_report_exception_handler.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,
@@ skipping 48 matching lines @@
   //!     object’s CrashReportDatabase.
   kern_return_t CatchMachException(
       exception_behavior_t behavior,
       exception_handler_t exception_port,
       thread_t thread,
       task_t task,
       exception_type_t exception,
       const mach_exception_data_type_t* code,
       mach_msg_type_number_t code_count,
       thread_state_flavor_t* flavor,
-      const natural_t* old_state,
+      ConstThreadState old_state,
       mach_msg_type_number_t old_state_count,
       thread_state_t new_state,
       mach_msg_type_number_t* new_state_count,
       const mach_msg_trailer_t* trailer,
       bool* destroy_complex_request) override;
 
  private:
   CrashReportDatabase* database_;  // weak
   CrashReportUploadThread* upload_thread_;  // weak
   const std::map<std::string, std::string>* process_annotations_;  // weak
 
   DISALLOW_COPY_AND_ASSIGN(CrashReportExceptionHandler);
 };
 
 }  // namespace crashpad
 
 #endif  // CRASHPAD_HANDLER_MAC_CRASH_REPORT_EXCEPTION_HANDLER_H_