Remove unused threading includes from FileReader.

Tidy up after r178426 and remove now-unused includes.

R=haraken@chromium.org
BUG=394828

Review URL: https://codereview.chromium.org/409433002

git-svn-id: svn://svn.chromium.org/blink/trunk@178525 bbb929c8-8fbe-4397-9dbb-9b2b20218538