Fix scoped_refptr construction from NULL

This is a precursor to adding a new scoped_refptr(std::nullptr_t)
constructor. The implicit conversion from NULL to scoped_refptr<T>
causes a compilation error with the new constructor. Replace NULL with
nullptr in any files where this is a problem.

This CL was uploaded by git cl split.

R=estade@chromium.org

Bug: 1018887
Change-Id: I29b84b7807d2d1b87cea6db9967986ac3aba900a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884998
Auto-Submit: kylechar <kylechar@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710057}

ash/system/message_center/message_center_ui_controller_unittest.cc

@@ -77,7 +77,7 @@ class MessageCenterUiControllerTest : public testing::Test {
   void TearDown() override {
     ui_controller_.reset();
     delegate_.reset();
-    message_center_ = NULL;
+    message_center_ = nullptr;
     message_center::MessageCenter::Shutdown();
   }
 
@@ -208,7 +208,7 @@ TEST_F(MessageCenterUiControllerTest,
           ASCIIToUTF16("Test Web Notification"),
           ASCIIToUTF16("Notification message body."), gfx::Image(),
           ASCIIToUTF16("www.test.org"), GURL(), DummyNotifierId(),
-          message_center::RichNotificationData(), NULL /* delegate */));
+          message_center::RichNotificationData(), nullptr /* delegate */));
   notification->SetSystemPriority();
   message_center_->AddNotification(std::move(notification));
 

ash/system/message_center/session_state_notification_blocker_unittest.cc

@@ -62,7 +62,7 @@ class SessionStateNotificationBlockerTest
         message_center::NOTIFICATION_TYPE_SIMPLE, "chromeos-id",
         UTF8ToUTF16("chromeos-title"), UTF8ToUTF16("chromeos-message"),
         gfx::Image(), UTF8ToUTF16("chromeos-source"), GURL(), notifier_id,
-        message_center::RichNotificationData(), NULL);
+        message_center::RichNotificationData(), nullptr);
     if (notifier_id.id == kNotifierSystemPriority)
       notification.set_priority(message_center::SYSTEM_PRIORITY);
     return blocker_->ShouldShowNotificationAsPopup(notification);