Fix typo in port.filesystem -> port._filesystem.

Sigh. Maybe later today I'll start working on more unit tests ...

TBR=peter@chromium.org

Review URL: https://codereview.chromium.org/78833004

git-svn-id: svn://svn.chromium.org/blink/trunk@162409 bbb929c8-8fbe-4397-9dbb-9b2b20218538