[Background Sync] Reland: Add field trial testing config.

This reverts commit dbf7046e760e372487d451a0c46335d391547874, which
originally reverted e14b9977feda8f681752e5fd37ae8bd8aca762f4
because of crbug.com/931705.

The bug has been fixed with a1d6f49fe0ae638e45a61f7edc46d3489741b4a7,
in https://chromium-review.googlesource.com/c/chromium/src/+/1472717,
thus this can be relanded.

Bug: 924490
Change-Id: If6c00873b0788d7786ffffa0dd6e1b61ffa72744
Reviewed-on: https://chromium-review.googlesource.com/c/1475467
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633439}
1 file changed