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When ServiceManager starts, it needs to post sequence tasks.
Therefore, we need to create a TaskScheduler and setup the
MessageLoop for the main thread.

Pariculaly for mac, the Mac registaration of NSApp now happens before
EarlyInitialization. The registration has been moved from
BrowserMainParts::PreMainMessageLoopStart() to ContentMainDelegate's
subclasses which implement ContentMainDelegate::PreMainMessageLoopStart().

This is required by a follow up CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1058131/

Change-Id: Ic67dace05b82fc44d6ab6d480ba0fecffe58fbf8
Reviewed-on: https://chromium-review.googlesource.com/1072387Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567244}