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

runtime/vm/os_thread.h

Index: runtime/vm/os_thread.h
diff --git a/runtime/vm/os_thread.h b/runtime/vm/os_thread.h
index da298316dd60bd29c673b2754365830eaa462ad1..41e77180ed74cf2dd256c4eede7dd3882b2faaab 100644
--- a/runtime/vm/os_thread.h
+++ b/runtime/vm/os_thread.h
@@ -108,6 +108,8 @@ class OSThread : public BaseThread {
     return true;
   }
 
+  static bool GetCurrentStackBounds(uword* lower, uword* upper);
+
   // Used to temporarily disable or enable thread interrupts.
   void DisableThreadInterrupts();
   void EnableThreadInterrupts();