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This will be used to properly remove observers during browser shutdown,
so that the omaha service will not DCHECK() due to leaving observers
registered.

Additionally renames methods from Initialize/ShutDown to Start/Stop.

BUG=996148

Change-Id: Ia0e23834f2ec3e11f4e04db2038cc2e144caa2b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1933735
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747302}