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Also updated OfflineGaiaTestMixin to use new JSChecker features.

Bug: 1110790
Change-Id: If30fde066b4768affc1ace615d2a745d791bbc1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418407
Commit-Queue: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808905}