Cleanup DOMFileSystem code

Remove Source/modules/filesystem/chromium directory and
make chromium-specific code default.

BUG=none

Review URL: https://codereview.chromium.org/14103005

git-svn-id: svn://svn.chromium.org/blink/trunk@148331 bbb929c8-8fbe-4397-9dbb-9b2b20218538