Implement NewScriptState::Scope

In preparation for removing ScriptState::Scope, ScriptScope and DOMRequestScope::Scope,
this CL adds NewScriptState::Scope.

BUG=357144

Review URL: https://codereview.chromium.org/224843010

git-svn-id: svn://svn.chromium.org/blink/trunk@170971 bbb929c8-8fbe-4397-9dbb-9b2b20218538