Use the async version of WebFileSystem::createFileWriter

Removing unnecessary callback chain.

(Some worker code still rely on the old createFileWriter, but
they're supposed to go away)

This depends on chrome-side patch:
https://codereview.chromium.org/18427005/

BUG=257349
TEST=no behavioral changes

Review URL: https://chromiumcodereview.appspot.com/18411005

git-svn-id: svn://svn.chromium.org/blink/trunk@154363 bbb929c8-8fbe-4397-9dbb-9b2b20218538