Refactoring only: Make the handling of PropertyType more explicit.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 29807da9f296e3e5e4bf33a89fa329793fffac0d..c1a87dbf824f4721f0e6a64031fcc4ca922b68d3 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -10406,10 +10406,11 @@ static MaybeObject* DebugLookupResultValue(Heap* heap,
     case CONSTANT_TRANSITION:
     case NULL_DESCRIPTOR:
       return heap->undefined_value();
-    default:
+    case HANDLER:
       UNREACHABLE();
+      return heap->undefined_value();
   }
-  UNREACHABLE();
+  UNREACHABLE();  // keep the compiler happy
   return heap->undefined_value();
 }