Did a bisect to find the CL that introduced the 1st such warning.

Bisect range https://chromium.googlesource.com/chromium/src/+log/4d1ac72d62bc7b7d70372878e2ab7c83af7a46a3..62d87e5dc7546545c602f1a8e03224b9548bef72

The culprit revision is most likely https://chromium.googlesource.com/chromium/src/+/311a7cec324e2c42ac0e201b7f7860a5a238c93f

Since then, other CLs have added more warnings (as shown in the screenshot).

@macourteau: Can you take a look? It seems that the code is unconditionally referring to a method that only exists on non-Mac platforms.

That is likely my CL. Will take a look.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/75c1ff3461e0602523a987de9aeddfc8a8a84422

commit 75c1ff3461e0602523a987de9aeddfc8a8a84422
Author: Marc-Antoine Courteau <macourteau@chromium.org>
Date: Fri Feb 02 02:22:43 2018

Remove observers that do not exist on macOS.

R=stevenjb

Bug:  808083 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I56dd69d92dd24393d63180449ab124ce93986652
Reviewed-on: https://chromium-review.googlesource.com/898078
Commit-Queue: Marc-Antoine Courteau <macourteau@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533922}
[modify] https://crrev.com/75c1ff3461e0602523a987de9aeddfc8a8a84422/chrome/browser/resources/settings/languages_page/languages_page.js