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This requires opening global_prefs a bit earlier. The code will release
global_prefs early if it determines qualification is necessary, and will
also release it before attempting to self-uninstall (this was an
oversight from earlier).

As a side-effect of this CL, a candidate will not be able to start
qualification while the active updater is running, since it cannot open
global_prefs in that case.

I also introduced a specific server exit code for when the server shuts
down post-qualification.

Bug: 1122739
Change-Id: Iad33f88ca438aee3fa2f5f0daf1105e2712baf0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2385795
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Auto-Submit: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803602}