Remove unnecessary static_casts to AsyncFileSystemCallbacks.

The specified objects are instances of derived classes of
AsyncFileSystemCallbacks, and PassRefPtr<> is clever enough to handle
inheritance.

BUG=none

Review URL: https://codereview.chromium.org/80113002

git-svn-id: svn://svn.chromium.org/blink/trunk@162438 bbb929c8-8fbe-4397-9dbb-9b2b20218538