DevTools: Switch service worker inspection to main thread.

BUG=404355
R=horo, yurys, pfeldman-OOO

Review URL: https://codereview.chromium.org/504223002

git-svn-id: svn://svn.chromium.org/blink/trunk@181172 bbb929c8-8fbe-4397-9dbb-9b2b20218538