Setup sandbox host in browser process early.

This is the precursor step before creating a thread to start the
ServiceManager before content_main_runner is running. It is because
no thread is expected to be when the zygote process is forked. In this
CL, we move the setup to ContentMainRunner::Initialize().

Bug: 740677,729596
Change-Id: I90e95cce8bc32be03c944aaa17f96c79d555e7cb
Reviewed-on: https://chromium-review.googlesource.com/999741
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550191}
3 files changed