VM: Teach GetAndValidateIsolateStackBounds(...) to fallback to OS thread stack bounds.

runtime/vm/os_thread_macos.cc

Index: runtime/vm/os_thread_macos.cc
diff --git a/runtime/vm/os_thread_macos.cc b/runtime/vm/os_thread_macos.cc
index 23fe4d7ca60b20edf593c57db6ac2a7655d02c82..bc33dc1d70231ddc1a2d57a8f188757c3a73bf3b 100644
--- a/runtime/vm/os_thread_macos.cc
+++ b/runtime/vm/os_thread_macos.cc
@@ -209,6 +209,13 @@ bool OSThread::Compare(ThreadId a, ThreadId b) {
 }
 
 
+bool OSThread::GetCurrentStackBounds(uword* lower, uword* upper) {
+  *upper = reinterpret_cast<uword>(pthread_get_stackaddr_np(pthread_self()));
+  *lower = *upper - pthread_get_stacksize_np(pthread_self());
+  return true;
+}
+
+
 Mutex::Mutex() {
   pthread_mutexattr_t attr;
   int result = pthread_mutexattr_init(&attr);