Using the per-revision bisect providing the bisect results,
Good build: 59.0.3056.0 (Revision: 460603).
Bad build : 59.0.3057.0 (Revision: 460966).

You are probably looking for a change made after 460936 (known good), but no later than 460937 (first known bad).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/d0203954892c94a15b4f5843bdc519b2be81f0bc..27793e2b00cd4f06e24cde9599be5693006ed02b

@scottchen: Could you please look into the issue, pardon me if it has nothing to do with your changes and if possible please assign it to concern owner.
Thank You.

for all dialogs: remove bottom border and reduce padding/font size of headers

for scrollable dialogs: remove the padding top/bottom of dialog bodies

assigning to bettes@ for specs

regarding "remove bottom border" for headers - should scrollable dialogs now have the border-top on the dialog body when there's stuff to scroll up to? currently it only adds a conditional border-bottom and not -top because it was assumed the header border's always going to be there.

Hey Scott,

To prepare us for future harmony design edits, I think it's best to remove the default bottom border for headers and allow the border to be drawn in conditionally for scrollable dialogs. 

So to answer your question in 4: I believe the answer is "yes." Header bottom borders will not exist in this new design so there should be conditional borders on top and bottom. 

- shrink close-x from 20px to 16px (36px ripple)
- Title text should be 15pt. (inspect shows 15.999px?)
- ensure title-text padding is 16 16 16 24
- With title text and new padding, title banner should equal 47px (15+16+16)

>> for scrollable dialogs: remove the padding top/bottom of dialog bodies

This is not true. Just cut it down from 12px to 8px. 

Deleted: Artboard 2.png 137 KB

	Artboard 2.png 137 KB View Download

I've made a CL that hits most of the stuff mentioned above, except for one:
Due to weirdness in how <paper-input> display error-messages, if we shrink the dialog body padding, then it will start to show unnecessary scroll-bars on many dialog body at medium font (it's currently already a problem for large font).

dpapad@ and I talked about paper-input's error-message display issue but we were not able to reach a good solution, so I'll be leaving dialog body padding at 12px for now to avoid regression bugs.

I believe we'll be okay with launching beta with 12px body padding, and we can address it later when we find a good solution to deal with paper-input error message.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fee40485e1568f0a7121576d722f2b66a5832c65

commit fee40485e1568f0a7121576d722f2b66a5832c65
Author: scottchen <scottchen@chromium.org>
Date: Mon Apr 10 19:54:34 2017

MD Settings: adjust dialog style and scroll border visual

BUG= 707631 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2795763003
Cr-Commit-Position: refs/heads/master@{#463373}

[modify] https://crrev.com/fee40485e1568f0a7121576d722f2b66a5832c65/chrome/browser/resources/settings/clear_browsing_data_dialog/clear_browsing_data_dialog.html
[modify] https://crrev.com/fee40485e1568f0a7121576d722f2b66a5832c65/chrome/browser/resources/settings/clear_browsing_data_dialog/history_deletion_dialog.html
[modify] https://crrev.com/fee40485e1568f0a7121576d722f2b66a5832c65/chrome/browser/resources/settings/printing_page/cups_add_printer_dialog.html
[modify] https://crrev.com/fee40485e1568f0a7121576d722f2b66a5832c65/chrome/browser/resources/settings/printing_page/cups_add_printer_dialog_util.html
[modify] https://crrev.com/fee40485e1568f0a7121576d722f2b66a5832c65/chrome/test/data/webui/cr_elements/cr_dialog_test.js
[modify] https://crrev.com/fee40485e1568f0a7121576d722f2b66a5832c65/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.html
[modify] https://crrev.com/fee40485e1568f0a7121576d722f2b66a5832c65/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js

Everything requested are in tip of tree, except for .body padding-top/bottom for reasons stated above. Marking as Fixed, and will open another bug for addressing paper-input error-message messing up layouts, and will include the body style in there.