[Background Sync] Revert changes to ChromeBackgroundService.

A recent CL added some logic based on a feature being enabled
in ChromeBackgroundService:
https://chromium-review.googlesource.com/c/chromium/src/+/1432840

However, since the native JNI library might not be loaded at this point,
the feature check can't be done -- it causes a crash.
This CL takes out logic to ChromeBackgroundService, to be re-introduced
when BackgroundTaskSchedulerForBackgroundSync feature is enabled
everywhere.

Bug: 924490, 932468
Change-Id: Ia1930c76a36e88a2a2bc53be96582123118ddc97
Reviewed-on: https://chromium-review.googlesource.com/c/1474655
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632720}