The reason for test failure is that latency of OpenTab() call reduced from ~1.2 sec to 0.5 sec. (https://cs.chromium.org/chromium/src/chrome/browser/sync/test/integration/single_client_sessions_sync_test.cc?q=single_client_sessions_sync_test.cc&sq=package:chromium&dr&l=380)

ListAccounts request starts in ProfileSyncService::OnEngineInitialized. Previously it was finishing before OpenTab completed and its effect was applied before sync cycle and cookie_jar_mismatch field in commit message reflected results of this request.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cf7529521cd4c7620abcf01bb7f43be833020f2f

commit cf7529521cd4c7620abcf01bb7f43be833020f2f
Author: Pavel Yatsuk <pavely@chromium.org>
Date: Fri Aug 04 19:23:30 2017

[Sync] Set cookie jar accounts before SingleClientSessionsSyncTest.CookieJarMismatch

The test was flaky because setting initial value of cookiejar mismatch depended
on the timing of ListAccounts request. Test started failing when latency of the
call to OpenTab reduced.

The change is to explicitly set initial cookie jar accounts to empty list in the
test.

BUG= 751419 
R=zea@chromium.org

Change-Id: Ia3eab3e9ceef7d53b127ba06ea29923d25e8a219
Reviewed-on: https://chromium-review.googlesource.com/602457
Reviewed-by: Nicolas Zea <zea@chromium.org>
Commit-Queue: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492098}
[modify] https://crrev.com/cf7529521cd4c7620abcf01bb7f43be833020f2f/chrome/browser/sync/test/integration/single_client_sessions_sync_test.cc