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ServiceWorker: Modernize http/tests/serviceworker/usecounter.html using async/await etc

This is a mechanical cleanup CL for making it easier to add new use counter tests.

Bug: 900444
Change-Id: Ib8d3c708c0f6ed1fd00b00bbcc9e873097cccd64
Reviewed-on: https://chromium-review.googlesource.com/c/1308240
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604166}