ServiceWorker: Fix conversion from Request headers to WebServiceWorkerRequest headers

The headers need to be coalesced when converting from one type to another.

A similar CL for WebServiceWorkerResponse headers is here: https://codereview.chromium.org/564773004/

BUG=415157

Review URL: https://codereview.chromium.org/578763003

git-svn-id: svn://svn.chromium.org/blink/trunk@182314 bbb929c8-8fbe-4397-9dbb-9b2b20218538