[DevTools] Simplify managing state in ServiceWorkerDevToolsAgentHost

Bug: 776009
Change-Id: I43af9e59b836e97524f057e4b60559b239f254bc
Reviewed-on: https://chromium-review.googlesource.com/862989
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529176}
12 files changed