Add use counter for web file system

This CL adds three use counters for file system to see how often the feature is
used on the main thread, in workers, and synchronously in workers.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/426343002

git-svn-id: svn://svn.chromium.org/blink/trunk@179264 bbb929c8-8fbe-4397-9dbb-9b2b20218538