Start TaskScheduler before creating BrowserMainLoop.

This is a follow up CL for:

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
Commit-Queue: Xi Han <>
Reviewed-by: John Abd-El-Malek <>
Reviewed-by: Gabriel Charette <>
Cr-Commit-Position: refs/heads/master@{#597255}
9 files changed