Move Service Worker %2f path validation logic from browser into Blink (1)

Move Service Worker %2f path validation logic from browser into Blink (1)

This patch adds an interface by which the path validation logic can be
kept in Chromium but called from within Blink. It will be eventually
used to ensure that navigator.serviceWorker.register() rejects with a
TypeError instead of a DOMException (see spec).

1. (Chromium) This CL.
2. (Blink) https://codereview.chromium.org/1260003003/
3. (Chromium) https://codereview.chromium.org/1256833004/

BUG=513622
R=nhiroki,falken

Review URL: https://codereview.chromium.org/1259213002

Cr-Commit-Position: refs/heads/master@{#342279}