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This is a follow up CL for: https://crrev.com/c/1115783.

In this CL, we use ScopedExecutionFence to prevents tasks running between the
peorid from BrowserMainRunner is instantiated to
BrowserMainLoop::CreateThreads() is called.

Bug: 846846
Change-Id: I9dec8415e45f901c5883c196ab867c0ea4928594
Reviewed-on: https://chromium-review.googlesource.com/c/1174955
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597255}