Rename NewScriptState to ScriptState

Now that the old ScriptState is gone, we can rename NewScriptState to ScriptState.

BUG=357144

Review URL: https://codereview.chromium.org/259863009

git-svn-id: svn://svn.chromium.org/blink/trunk@172857 bbb929c8-8fbe-4397-9dbb-9b2b20218538