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..978dff91083151192c17a2a105e6d37faa8e4aae 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -4892,6 +4892,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_StoreArrayLiteralElement) {
 // Check whether debugger and is about to step into the callback that is passed
 // to a built-in function such as Array.forEach.
 RUNTIME_FUNCTION(MaybeObject*, Runtime_DebugCallbackSupportsStepping) {
+#ifdef ENABLE_DEBUGGER_SUPPORT
   if (!isolate->IsDebuggerActive()) return isolate->heap()->false_value();
   CONVERT_ARG_CHECKED(Object, callback, 0);
   // We do not step into the callback if it's a builtin or not even a function.
@@ -4899,12 +4900,16 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_DebugCallbackSupportsStepping) {
     return isolate->heap()->false_value();
   }
   return isolate->heap()->true_value();
+#else
+  return isolate->heap()->false_value();
+#endif  // ENABLE_DEBUGGER_SUPPORT
 }
 
 
 // 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);
@@ -4914,7 +4919,8 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_DebugPrepareStepInIfStepping) {
   // again, we need to clear the step out at this point.
   debug->ClearStepOut();
   debug->FloodWithOneShot(callback);
-  return NULL;
+#endif  // ENABLE_DEBUGGER_SUPPORT
+  return isolate->heap()->undefined_value();
 }