ServiceWorker: Spec alignment tweaks for Request/Response objects

Request.method should be ByteString (now supported)
Response.status should be assignable
Response.statusText should be assignable
Response.statusText should be ByteString (now supported)

And added tests for assignable properties/invalid ByteStrings.

R=falken@chromium.org,horo@chromium.org

Review URL: https://codereview.chromium.org/337793003

git-svn-id: svn://svn.chromium.org/blink/trunk@176148 bbb929c8-8fbe-4397-9dbb-9b2b20218538