Refine wpt/service-workers/service-worker/clients-get.https.html

Used async functions, arrow functions, and const, and added some comments.

Bug: 778497
Change-Id: I03f23c6adddbb0e317edb197dc8af197597b2381
Reviewed-on: https://chromium-review.googlesource.com/c/1343416
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609693}
5 files changed