Decouple FileSystem and WebSQL callbacks from VoidCallback and make it non-refcounted

Create two new VoidCallback clones in modules/filesystem and modules/webdatabase,
and make the original non-refcounted. This will meet two goals:
  - The only code left using refcounted callbacks are in FileSystem and WebSQL
  - Evolving those two modules' use of callbacks into the modern age should be
    easier now that they're isolated from each other.

R=abarth@chromium.org
BUG=323681

Review URL: https://codereview.chromium.org/103083002

git-svn-id: svn://svn.chromium.org/blink/trunk@163132 bbb929c8-8fbe-4397-9dbb-9b2b20218538