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Clean up style in testharnessreport.js

This CL changes testharnessreport.js to adhere closer to
the style guide:
https://chromium.googlesource.com/chromium/src/+/master/styleguide/web/web.md#JavaScript

In this CL:
 - Wrap long lines
 - Make quotes consistent
 - Replaces var with let/const
 - Update some comments

This should not change behavior.

Change-Id: I1df1b42fd74333bba392eb8f3a91a15b4eb4d4fd
Reviewed-on: https://chromium-review.googlesource.com/600769
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491878}