Oilpan: Prepare moving DateTimeChooser to Oilpan

Source/core/loader/EmptyClients.cpp

Index: Source/core/loader/EmptyClients.cpp
diff --git a/Source/core/loader/EmptyClients.cpp b/Source/core/loader/EmptyClients.cpp
index 10693cafeffda1ea95ca9d30ed2963114f5429af..db5b8aa7068fd70f3e01bff0b05f7f130cc6540c 100644
--- a/Source/core/loader/EmptyClients.cpp
+++ b/Source/core/loader/EmptyClients.cpp
@@ -86,9 +86,9 @@ PassOwnPtr<ColorChooser> EmptyChromeClient::createColorChooser(LocalFrame*, Colo
     return nullptr;
 }
 
-PassRefPtr<DateTimeChooser> EmptyChromeClient::openDateTimeChooser(DateTimeChooserClient*, const DateTimeChooserParameters&)
+PassRefPtrWillBeRawPtr<DateTimeChooser> EmptyChromeClient::openDateTimeChooser(DateTimeChooserClient*, const DateTimeChooserParameters&)
 {
-    return PassRefPtr<DateTimeChooser>();
+    return nullptr;
 }
 
 void EmptyChromeClient::openTextDataListChooser(HTMLInputElement&)