[Pref sync] Fix flaky test TwoClientPreferencesSyncTest.Sanity

This CL introduces an additional waiting in a test to avoid flakiness.

Bug: 956043
Change-Id: Ibef2e32fbb6a82350137cb506519dd8fcc5b84e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583734
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654456}
1 file changed