Revert "Make ScriptLoader into a ResourceOwner"

This reverts r173566. It's believed to be the cause of a lot of crashes.
See the bug for more details.

BUG=372242
TBR=morrita@chromium.org

Review URL: https://codereview.chromium.org/271703011

git-svn-id: svn://svn.chromium.org/blink/trunk@173996 bbb929c8-8fbe-4397-9dbb-9b2b20218538