Fix assigning NULL to scoped_refptr

Update code where NULL or 0 is assigned to a scoped_refptr to reset it. Just
call reset() instead. If operator=(std::nullptr_t) is added these assignments
become ambiguous.

This CL was uploaded by git cl split.

R=skuhne@chromium.org

Bug: 1024981
Change-Id: I584886409b60f2cae2c868cf52ff72186a2af041
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918324
Auto-Submit: kylechar <kylechar@chromium.org>
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Commit-Queue: Stefan Kuhne <skuhne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715579}

ash/system/bluetooth/bluetooth_notification_controller.cc

@@ -164,7 +164,7 @@ BluetoothNotificationController::~BluetoothNotificationController() {
   if (adapter_.get()) {
     adapter_->RemoveObserver(this);
     adapter_->RemovePairingDelegate(this);
-    adapter_ = NULL;
+    adapter_.reset();
   }
 }