Make ScriptLoader into a ResourceOwner

This change turns ScriptLoader a ResourceOwner, that ensures that
it subscribes Resource if the owner has a reference to it, and that
it unsubscribes the Resource otherwise. Before this change,
There are some situation where ScriptLoader has a reference to Resource
but doesn't subscribe it, that is error prone and left deleted instance
being subscribed. ResourceOwner guarantees reference and subscription
consistency.

TEST=script-cancel-crash.html
BUG=368551
R=eseidel@chromium.org, japhet@chromium.org, esprehn@chromium.org

Review URL: https://codereview.chromium.org/271533002

git-svn-id: svn://svn.chromium.org/blink/trunk@173566 bbb929c8-8fbe-4397-9dbb-9b2b20218538