Revert 174019 "Make ScriptLoader into a ResourceOwner"

> 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.
> 
> This is the second attempt to land this change. The original one was
> reverted at r173996 because it caused a lot of crashes. These crahses
> shouldn't happen with this revised version.
> 
> TEST=script-cancel-crash.html
> BUG=368551,372242
> TBR=eseidel@chromium.org, japhet@chromium.org, dglazkov@chromium.org
> 
> Review URL: https://codereview.chromium.org/288673002

TBR=morrita@chromium.org

Review URL: https://codereview.chromium.org/283333002

git-svn-id: svn://svn.chromium.org/blink/trunk@174071 bbb929c8-8fbe-4397-9dbb-9b2b20218538