Add ConstThreadState to mach_extensions.h and use it everywhere

handler/mac/crash_report_exception_handler.cc

 // 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 70 matching lines @@
 
 kern_return_t CrashReportExceptionHandler::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) {
   *destroy_complex_request = true;
 
   // The expected behavior is EXCEPTION_STATE_IDENTITY | MACH_EXCEPTION_CODES,
   // but it’s possible to deal with any exception behavior as long as it
   // carries identity information (valid thread and task ports).
@@ skipping 153 matching lines @@
 
   if (!forwarded) {
     ExcServerCopyState(
         behavior, old_state, old_state_count, new_state, new_state_count);
   }
 
   return ExcServerSuccessfulReturnValue(behavior, false);
 }
 
 }  // namespace crashpad