[Isolates] Fix debugger and termination APIs callable from other threads.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 91bdb8102d5da6a769404d3b0ccd9eda091e5d71..b2d8d3982f5463720b310011ae20f54f9257a4e0 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -431,6 +431,14 @@ class Isolate {
   // Safe to call multiple times.
   static void EnsureDefaultIsolate();
 
+  // Get the debugger from the default isolate. Preinitializes the
+  // default isolate if needed.
+  static Debugger* GetDefaultIsolateDebugger();
+
+  // Get the stack guard from the default isolate. Preinitializes the
+  // default isolate if needed.
+  static StackGuard* GetDefaultIsolateStackGuard();
+
   // Returns the key used to store the pointer to the current isolate.
   // Used internally for V8 threads that do not execute JavaScript but still
   // are part of the domain of an isolate (like the context switcher).