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The thread ServiceWorkerContextCore lives on (the "core thread") will
move from the IO thread to the UI thread when the ServiceWorkerOnUI
feature is enabled.

This makes most tests involving extension service workers to pass.

Bug: 824858, 984522
Change-Id: Ide0d60cbe9be15ea6149d29761a50506ac4376fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1763560
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695394}