Thanks for finding this. Looks like you've already started a CL on this, so I'll wait till you are ready for review.

Of course. crrev.com/c/1057895 is ready for review. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/43d0900afe24403be4e9de6ca95b83ce40d71c64

commit 43d0900afe24403be4e9de6ca95b83ce40d71c64
Author: Moe Ahmadi <mahmadi@chromium.org>
Date: Tue May 15 13:20:31 2018

[IOS] Fixes infobar action button's title font changing size when tapped

Bug:  842746 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ie69687c50368a84ae97f1d648ce33645ce94402b
Reviewed-on: https://chromium-review.googlesource.com/1057895
Commit-Queue: Peter Lee <pkl@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558683}
[modify] https://crrev.com/43d0900afe24403be4e9de6ca95b83ce40d71c64/ios/chrome/browser/ui/infobars/confirm_infobar_view.mm