Revert r161710 and r161714

Revert following changes:
- r161714: "Remove wrong overload."
- r161710: "Refactoring: Extract ExecutionContextTaskRunner from Document."

It turns out ExecutionContext needs its own ref count after all
because Worker refers the context and the primary owner of the context
cannot delete it by its own right.

TBR=abarth
BUG=305497

Review URL: https://codereview.chromium.org/70073002

git-svn-id: svn://svn.chromium.org/blink/trunk@161774 bbb929c8-8fbe-4397-9dbb-9b2b20218538