Fix compilation when disabling debugger support.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 74f3e43ada4e995fd26ba6c038a1ac008598db16..1db187df9586d64d442582a95cda704b37358c08 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -4905,6 +4905,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_DebugCallbackSupportsStepping) {
 // Set one shot breakpoints for the callback function that is passed to a
 // built-in function such as Array.forEach to enable stepping into the callback.
 RUNTIME_FUNCTION(MaybeObject*, Runtime_DebugPrepareStepInIfStepping) {
+#ifdef ENABLE_DEBUGGER_SUPPORT
   Debug* debug = isolate->debug();
   if (!debug->IsStepping()) return NULL;
   CONVERT_ARG_HANDLE_CHECKED(JSFunction, callback, 0);
@@ -4914,6 +4915,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_DebugPrepareStepInIfStepping) {
   // again, we need to clear the step out at this point.
   debug->ClearStepOut();
   debug->FloodWithOneShot(callback);
+#endif
   return NULL;
 }