NewScriptState should own V8PerContextData

- I'm making NewScriptState a default class that piggybacks all states about script execution. To achieve that goal, NewScriptState should own V8PerContextData.

- This CL changes V8WindowShell and WorkerScriptController so that they own NewScriptState instead of V8PerContextData.

BUG=357144

Review URL: https://codereview.chromium.org/217423020

git-svn-id: svn://svn.chromium.org/blink/trunk@170747 bbb929c8-8fbe-4397-9dbb-9b2b20218538