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The thread ServiceWorkerContextCore lives on (the "core thread")
will move from the IO thread to the UI thread.

This makes some code used in navigation be core thread-aware instead
of assuming the IO thread. This allows many content_browsertests
for ServiceWorker to pass when ServiceWorkerOnUI is enabled.

Bug: 824858
Change-Id: I3e70cf88f615e77a8f45370eea002b6b344e1511
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760829
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688284}