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This relands commit aa60c219. The original CL
(https://crrev.com/c/1113802) got reverted becuase FeatureList and field
trials are not setup properly in early startup in Android WebView, but
ServiceMangerContext has checked features. See crbug.com/899376. The fix is
in a separate CL: https://crrev.com/c/1305876. Another related precursor CL
is: https://crrev.com/c/1308096.

This CL is an exact copy of the original CL.

The original cl description is:
This CL instantiates the ServiceManagerContext before creating
the BrowserMainRunner. It splits the startup path into two,
with/without starting the full browser. The changes are implemented
behind a flag "allow-start-service-manager-only".

Bug: 846846,729596
TBR: jam@chromium.org
Change-Id: I5214af850d4ef256c9d223db059ed009a42de714
Reviewed-on: https://chromium-review.googlesource.com/c/1305822
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604969}