ServiceWorker: UMA histogram to track the size of Service Worker scripts

This adds ServiceWorker.ScriptSize histogram that records the length of
Service Worker scripts.

BUG=408451

Review URL: https://codereview.chromium.org/524553002

git-svn-id: svn://svn.chromium.org/blink/trunk@181289 bbb929c8-8fbe-4397-9dbb-9b2b20218538