Remove use of "register" keyword, deprecated in C++17

"register" as a storage class specifier has been deprecated since C++11,
and has been removed from C++17 while remaining a reserved word. See
C++17 5.11 and C.4.3.

Change-Id: I2dbab8a7061cb680d902644d39ea1a7fbc930e5c
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/1749329
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/src/client/linux/minidump_writer/linux_dumper_unittest_helper.cc b/src/client/linux/minidump_writer/linux_dumper_unittest_helper.cc
index 3ad48e5..2dfe09b 100644
--- a/src/client/linux/minidump_writer/linux_dumper_unittest_helper.cc
+++ b/src/client/linux/minidump_writer/linux_dumper_unittest_helper.cc
@@ -65,7 +65,7 @@
     perror("ERROR: parent notification failed");
     return NULL;
   }
-  register volatile pid_t *thread_id_ptr asm(TID_PTR_REGISTER) = thread_id;
+  volatile pid_t *thread_id_ptr asm(TID_PTR_REGISTER) = thread_id;
   while (true)
     asm volatile ("" : : "r" (thread_id_ptr));
   return NULL;