Fix at https://chromium-review.googlesource.com/c/chromium/src/+/1287258. It only happens with Polymer 2 (which is now turned on by default on ToT).

 Issue 896596  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5c79ead06c24400f12c150529a5ec55060577f87

commit 5c79ead06c24400f12c150529a5ec55060577f87
Author: dpapad <dpapad@chromium.org>
Date: Thu Oct 18 19:39:03 2018

Fix sync-confirmation dialog styling in Polymer 2.

Need to wrap <style is="custom-style"> with <custom-style> such that styling
works both in Polymer 1 and Polymer 2.

Bug: 739532, 896507 
Change-Id: I40e4a1fa6ac071cc31268c5e86fa13a023a59490
Reviewed-on: https://chromium-review.googlesource.com/c/1287258
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600868}
[modify] https://crrev.com/5c79ead06c24400f12c150529a5ec55060577f87/chrome/browser/resources/signin/sync_confirmation/sync_confirmation.html

 Issue 896595  has been merged into this issue.