Button height 36px + top margin 4px + bottom margin 4px = 44 px. Title height 47px. 
To be center aligned with title, there'll be a 1~2px offset comparing to spec, otherwise the math won't add up.

I wasn't considering alignment with a title a requirement actually. Something like the following?

Deleted: close-x.png 84.5 KB

	close-x.png 84.5 KB View Download

Discarding center alignment and having a fixed 4px margin top/right in very large fonts will look like the attached screenshot. Please let me know if thats what we want and I can make the change.

Deleted: Screenshot from 2017-04-14 11:00:37.png 17.8 KB

	Screenshot from 2017-04-14 11:00:37.png 17.8 KB View Download

Is there a reason to evaluate this for large fonts only?

The larger the font, the more vertically off-centered it would be comparing to the title font. 
You did already mention that center-alignment wasn't a requirement, I just wanted to confirm that the result actually does look okay to you.

I see. Yeah your screenshot in #3 lgtm. Thanks for clarifying dude!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f6cbd1a7c6fc9970840ad5242dad4cfb55753630

commit f6cbd1a7c6fc9970840ad5242dad4cfb55753630
Author: scottchen <scottchen@chromium.org>
Date: Thu Apr 20 21:34:23 2017

WebUI: dialog close button should have fixed 4px top margin

BUG= 711425 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2826763002
Cr-Commit-Position: refs/heads/master@{#466146}

[modify] https://crrev.com/f6cbd1a7c6fc9970840ad5242dad4cfb55753630/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.html