Remove ScriptState::current from modules/serviceworker/

ScriptState::current should be called only in the binding layer.

BUG=357144

Review URL: https://codereview.chromium.org/307243003

git-svn-id: svn://svn.chromium.org/blink/trunk@175290 bbb929c8-8fbe-4397-9dbb-9b2b20218538