Cleanup LocalFileSystem (part 1)

- No LocalFileSystem singleton any more
- Introduce LocalFileSystemClient interface
- Remove LocalFileSystemChromium and move the implementation into
  two client impl files: LocalFileSystemClientImpl and
  WorkerLocalFileSystemClientImpl.

LocalFileSystem supplements Page, and
WorkerLocalFileSystem supplements WorkerClients.

BUG=257349
TEST=fast/filesystem/workers/*

Review URL: https://chromiumcodereview.appspot.com/19466007

git-svn-id: svn://svn.chromium.org/blink/trunk@154750 bbb929c8-8fbe-4397-9dbb-9b2b20218538