ScheduledAction should hold NewScriptState instead of ScopedPersistent<v8::Context>

For consistency, V8 Proxy objects should hold NewScriptState instead of other structures such as ScopedPersistent<v8::Context>, DOMWrapperWorld etc.

BUG=357144

Review URL: https://codereview.chromium.org/256783007

git-svn-id: svn://svn.chromium.org/blink/trunk@172744 bbb929c8-8fbe-4397-9dbb-9b2b20218538