mach_port_deallocate() the result of mach_thread_self(), which obtains a port send right.

base/threading/platform_thread_posix.cc

Index: base/threading/platform_thread_posix.cc
diff --git a/base/threading/platform_thread_posix.cc b/base/threading/platform_thread_posix.cc
index 0196ff27fd844839aaf90507b99427919fd80303..7eda4e97c51169bc28b7fef49ca7466e1ed4eaf2 100644
--- a/base/threading/platform_thread_posix.cc
+++ b/base/threading/platform_thread_posix.cc
@@ -133,7 +133,9 @@ PlatformThreadId PlatformThread::CurrentId() {
   // Pthreads doesn't have the concept of a thread ID, so we have to reach down
   // into the kernel.
 #if defined(OS_MACOSX)
-  return mach_thread_self();
+  mach_port_t port = mach_thread_self();
+  mach_port_deallocate(mach_task_self(), port);
+  return port;
 #elif defined(OS_LINUX)
   return syscall(__NR_gettid);
 #elif defined(OS_ANDROID)