[runtime] cleanup: use the factory() for handlified values

BUG=

Review-Url: https://codereview.chromium.org/2163823004
Cr-Commit-Position: refs/heads/master@{#37981}

src/runtime/runtime-debug.cc

@@ -1582,7 +1582,7 @@ static Handle<Object> GetJSPositionInfo(Handle<Script> script, int position,
                                         Isolate* isolate) {
   Script::PositionInfo info;
   if (!script->GetPositionInfo(position, &info, offset_flag)) {
-    return handle(isolate->heap()->null_value(), isolate);
+    return isolate->factory()->null_value();
   }
 
   Handle<String> source = handle(String::cast(script->source()), isolate);

src/runtime/runtime-generator.cc

@@ -27,7 +27,7 @@ RUNTIME_FUNCTION(Runtime_CreateJSGeneratorObject) {
     operand_stack = isolate->factory()->NewFixedArray(size);
   } else {
     // Old-style generators.
-    operand_stack = handle(isolate->heap()->empty_fixed_array());
+    operand_stack = isolate->factory()->empty_fixed_array();
   }
 
   Handle<JSGeneratorObject> generator =