Remove ScriptState::current() from StorageQuota

ScriptState::current() should be called only from the binding layer. This CL removes it from StorageQuota.

No change in behavior.

BUG=357144

Review URL: https://codereview.chromium.org/297073002

git-svn-id: svn://svn.chromium.org/blink/trunk@174785 bbb929c8-8fbe-4397-9dbb-9b2b20218538