Revert of Make the EarlyInitStartup Code Default (patchset #1 id:1 of https://codereview.chromium.org/1453923002/ )
Revert of Make the EarlyInitStartup Code Default (patchset #1 id:1 of https://codereview.chromium.org/1453923002/ )

Reason for revert:
Reverting - Looks like we can't easily get perf data from the bots.

Original issue's description:
> Make the EarlyInitStartup Code Default
>
> We're satisfied with the performance characteristics of running Extension
> Feature Initialization on a background thread that we'd like to do it
> all the time now.
>
> BUG=470743
>
> Committed: https://crrev.com/697fb4718088e9b87a30f21f57168af70ef273ae
> Cr-Commit-Position: refs/heads/master@{#360352}
>
> Committed: https://crrev.com/9b13cbdc3205421a28bd583878c1108ed0eaead0
> Cr-Commit-Position: refs/heads/master@{#378308}

TBR=jochen@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=470743

Review URL: https://codereview.chromium.org/1749213005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378782}

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