Implement support for link type serviceworker in link elements.

      After a previous CL landed support for link rel=serviceworker in Link HTTP
headers, this CL adds similar support to link elements as specified in
https://github.com/slightlyoff/ServiceWorker/pull/828

BUG=582310

Review URL: https://codereview.chromium.org/1781783002

Cr-Commit-Position: refs/heads/master@{#382733}