Return Object* instead of MaybeObject* from runtime calls.

src/execution.cc

Index: src/execution.cc
diff --git a/src/execution.cc b/src/execution.cc
index eb1fa4c1d9aba0af3ec93a4744dbd23848542179..d52d471b1bd21c675d5b4d08fad4e513ad0f8750 100644
--- a/src/execution.cc
+++ b/src/execution.cc
@@ -54,7 +54,7 @@ MUST_USE_RESULT static MaybeHandle<Object> Invoke(
   }
 
   // Placeholder for return value.
-  MaybeObject* value = reinterpret_cast<Object*>(kZapValue);
+  Object* value = NULL;
 
   typedef Object* (*JSEntryFunction)(byte* entry,
                                      Object* function,
@@ -941,7 +941,7 @@ void Execution::ProcessDebugMessages(Isolate* isolate,
 }
 #endif
 
-MaybeObject* Execution::HandleStackGuardInterrupt(Isolate* isolate) {
+Object* Execution::HandleStackGuardInterrupt(Isolate* isolate) {
   StackGuard* stack_guard = isolate->stack_guard();
   if (stack_guard->ShouldPostponeInterrupts()) {
     return isolate->heap()->undefined_value();