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- Make ShutdownType an enum class.
- Introduce browser_shutdown::HasShutdownStarted as a convenience.
- Move some Win-specific relaunch code from AttemptRestart, which runs
  fairly early in a restart that may never actually happen, into
  upgrade_util::RelaunchChromeBrowserImpl, which performs the actual
  restart.
- Switch from UMA macros to functions for one-shot metrics.

BUG=958865

Change-Id: I75cd08c712d8bb7c674b4b5e413d79abc874bc39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1910098Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715730}