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This allows debugging the updater engine without the hassles of dealing
with the client-server(service) RPC multiprocess model.

Consider implementing the mode for macOS too, if it is valuable.

Run updater --single-process to enter this mode.

Bug: 1060800
Change-Id: I63c198e1af454ec885b7248ec86d935351e79bea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2099603Reviewed-by: S. Ganesh <ganesh@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749432}