DeleteDuringNotify provokes a WeakPtr thread-check failure.

Looking at the test, it's unclear how it ever passes, since it creates the "Deleter" delegate on the test thread, binds a callback to be invoked[1] by the FilePathWatcher on the file thread, and then deletes[2] it on the test thread - #1 and #2 are on different threads, which WeakPtr won't allow.