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Google-internal design doc:
https://docs.google.com/document/d/1X6Xoz9hl3qoAj3EhAKcvZ6hWLVcY_QpQgmUexpgDHKA/edit?usp=sharing

Move prefs creation from AwBrowserContext in PreMainMessageLoopRun to
AwBrowserMainParts in PreCreateThreads. This way, variations can use the
same pref service as the rest of WebView, while still being initialized
as early as possible. (See BrowserMainLoop::CreateStartupTasks() for an
overview of the startup sequence.)

Before, AwBrowserContext created and owned PrefService and
BrowserPolicyConnector. Now, AwBrowserMainParts creates them in
PreCreateThreads, and owns them until PreMainMessageLoopRun, at which
point ownership is transferred to AwBrowserContext.

BUG=866722

Change-Id: Ie8eb6df60bd4a0f872d8c6401cc14ff0bca38368
Reviewed-on: https://chromium-review.googlesource.com/1212268Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589665}