Only accept Functions as callbacks for SQLTransaction

This matches the spec for WebSQL (which uses [Callback=FunctionOnly]).
However, there is some compatibility risk, as this does not match Safari.
But it would make all IDL callbacks in Blink consistent in accepting only
functions.

R=eseidel@chromium.org, haraken@chromium.org, michaeln@chromium.org

Review URL: https://codereview.chromium.org/109053002

git-svn-id: svn://svn.chromium.org/blink/trunk@163653 bbb929c8-8fbe-4397-9dbb-9b2b20218538