WebPrivatePtr should use WTF::{,de}refPtrIfNotNull for consistency

Before this patch, WebPrivatePtr implemented null checks in its own.
This is a bit cumbersome when adding custom debugging hooks to ref/derefs.

BUG=None

Review URL: https://codereview.chromium.org/375813002

git-svn-id: svn://svn.chromium.org/blink/trunk@177714 bbb929c8-8fbe-4397-9dbb-9b2b20218538