Cleanup: Combine Worker-specific FileSystem client implementation into the document one

Looks like technically one class can derive from multiple Supplement<T>
(with different T).

This combines WorkerLocalFileSystem into LocalFileSystem, and
WorkerFileSystemClient into LocalFileSystemClient.

BUG=none

Review URL: https://codereview.chromium.org/64293002

git-svn-id: svn://svn.chromium.org/blink/trunk@161548 bbb929c8-8fbe-4397-9dbb-9b2b20218538