[ServiceWorker] Minor cleanup for ServiceWorkerContextClient

BUG=

Change-Id: Ie5e7c172b3fe424edb8eae481a4f5245a0ee1a3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1612350
Commit-Queue: Leon Han <leon.han@intel.com>
Auto-Submit: Leon Han <leon.han@intel.com>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659811}
4 files changed