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With "allow-start-service-manager-only" flag, we can start the service
manager without starting the remaining parts of the browser,
particularly the browser main loop. Later, we could start the full
browser with the service manager running. Currently, both code path go
through service_manager::main(). Therefore, we need to avoid calling the
initialization in service_manager::main() multiple times.

In this CL:
1. service_manager::main() checks its static bool flag to decide
   whether initialization should be called.
2. Plumbs the start-service-manager-only flag to the ContentMainRunner
   who starts the browser main loop.

This CL impacts Android only for now. It follows up CL:
 https://crrev.com/c/1093303.

BUG=854209, 846846

Change-Id: I7fcfb804ce7f3065da7df12947674344fe2f56d6
Reviewed-on: https://chromium-review.googlesource.com/1114077
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574169}