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This reverts commit 63964fe6.

Reason for revert: Failed to compile on linux-chromeos-chrome. https://ci.chromium.org/p/chrome/builders/ci/linux-chromeos-chrome/3680

Original change's description:
> Refactor the session managers by using SessionManagerObserver
> 
> Use a new class 'UserSessionInitializer' to handle common initialization
> logics in 'ChromeSessionManager' and 'UserSessionManager'. Whenever
> profiles are loaded, corresponding methods in the observer will be
> triggered.
> 
> Bug: 977489
> Change-Id: I24259636906d80cfbec48268eb0c68aa851a4df7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2044278
> Commit-Queue: Anqing Zhao <anqing@google.com>
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#746258}

TBR=xiyuan@chromium.org,anqing@google.com

Change-Id: Iab6d9b65871a4866dabfb20d66bacaca6df0cedf
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 977489
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083780Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746267}