Add ConstThreadState to mach_extensions.h and use it everywhere.

TEST=everything
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Review URL: https://codereview.chromium.org/1058523002
diff --git a/handler/mac/crash_report_exception_handler.h b/handler/mac/crash_report_exception_handler.h
index 01b9550..67a523a 100644
--- a/handler/mac/crash_report_exception_handler.h
+++ b/handler/mac/crash_report_exception_handler.h
@@ -66,7 +66,7 @@
       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,