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This is required to remove the |local_state_task_runner| member
of BrowserProcessImpl only used to implicitly wait on pref store.
Ref. https://crrev.com/c/1163628.
Synchronous callback semantics are required on EndSession() as a nested
RunLoop is not suitable to observe a reply.
https://chromium-review.googlesource.com/c/chromium/src/+/1163628/8/chrome/browser/browser_process_impl.cc#594

Also implemented in services/preferences' SegregatedPrefStore but
not in the Mojom interface where I don't think it's used yet? Or if
it is then it was already wrong as |local_state_task_runner| is
decoupled from that Mojom. The DCHECK will tell and make this future
proof.

Bug: 848615
Cq-Include-Trybots: luci.chromium.try:linux_mojo;master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ie72f2d30d30bfa7f96a04d780d1591949a173b78
Reviewed-on: https://chromium-review.googlesource.com/1164522Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581324}