[DevTools] Hide service workers filter when no SW

Bug: 783113
Change-Id: I8fa7f12d8908615ea9b5bdc0bc3baba646ccec04
Reviewed-on: https://chromium-review.googlesource.com/778047


Commit-Queue: Eugene Ostroukhov <eostroukhov@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518296}