ServiceWorker: Update IDLs to use ScalarValueString

Based on the IDLs in the SW spec and now Fetch spec, use
ScalarValueString as appropriate.

Also, add cases to the overload resolution implementation for ByteString
(and ScalarValueString, though the spec hasn't caught up yet).

R=nbarth@chromium.org,haraken@chromium.org,horo@chromium.org
BUG=379009

Review URL: https://codereview.chromium.org/340943002

git-svn-id: svn://svn.chromium.org/blink/trunk@176531 bbb929c8-8fbe-4397-9dbb-9b2b20218538