[FeatureList] Create default FeatureList early in PostEarlyInitialization.

This CL add a default implementation of
ContentMainDelegate::PostEarlyInitialization() to create FeatureList,
and remove the creation in BrowserMainLoop which happens after staring
the TaskScheduler in CL (https://crrev.com/c/1174955). This
fix browser tests failures when the TaskScheduler is starting before
FeatureList is created.

Bug: 848615, 729596
Change-Id: I9f9e85c785d41ad518db4170db53c4fc1d57e90d
Reviewed-on: https://chromium-review.googlesource.com/c/1228615
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596712}
25 files changed