Make WebFileSystemCallbacks not self-destruct, deprecate AsyncFileSystem

The last piece of cleanup.

Depends on chromium-side change (with ifdef):
https://codereview.chromium.org/23762002/

BUG=257349

Review URL: https://chromiumcodereview.appspot.com/23704004

git-svn-id: svn://svn.chromium.org/blink/trunk@157378 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Made pad-gesture-fling layout test deterministic

BUG=261277

Review URL: https://chromiumcodereview.appspot.com/23711005

git-svn-id: svn://svn.chromium.org/blink/trunk@157379 bbb929c8-8fbe-4397-9dbb-9b2b20218538