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

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 60077e495277f4b05552bf16963bffc59dc17601..171e9f38351a5bb0439ebc8e084db9fbabc30c0b 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -254,6 +254,18 @@ void Isolate::EnsureDefaultIsolate() {
 }
 
 
+Debugger* Isolate::GetDefaultIsolateDebugger() {
+  EnsureDefaultIsolate();
+  return default_isolate_->debugger();
+}
+
+
+StackGuard* Isolate::GetDefaultIsolateStackGuard() {
+  EnsureDefaultIsolate();
+  return default_isolate_->stack_guard();
+}
+
+
 void Isolate::EnterDefaultIsolate() {
   EnsureDefaultIsolate();
   ASSERT(default_isolate_ != NULL);