Small correction in steps:
 After emulation scroll down page and type something in Sign Up for Great Offers & Values from Costco.com text field instead of email address field.

Using the per-revision bisect providing the bisect results,
Good build: 55.0.2860.0 (Revision: 418438).
Bad build: 55.0.2861.0 (Revision: 418732).

You are probably looking for a change made after 418694 (known good), but no later than 418695 (first known bad).
CHANGELOG URL:
 https://chromium.googlesource.com/chromium/src/+log/9308976b1622d17756637bc83ffd2e3d8fd12862..982432d01a43f9bede4b7b442f9b7d5ffa8a20c2

From the CL above, assigning the issue to the concern owner --
@cbiesinger -- 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.

Works fine for me on Mac OS 10.11.6, chrome version: 55.0.2864.0.

I'm sorry, but it's not possible that my change caused this bug. The code I changed is only used with a special command-line flag. Can you try bisecting again?

Using the per-revision bisect providing the rebisect result,
Good build: 55.0.2859.0 (Revision: 418117).
Bad build: 55.0.2861.0 (Revision: 418732).

You are probably looking for a change made after 418694 (known good), but no later than 418695 (first known bad).

CHANGELOG URL:
https://chromium.googlesource.com/chromium/src/+log/d19c087b9bfdf76a8af91e2f0a0e791bf2ffcafa..db00688793f63e1f3ccd71f883ec3129bd169abe

From the CL above, assigning the issue to the concern owner --
@loyso - Could you please check whether this is caused with respect to your change, if not please help us in assigning it to the right owner.

Thanks!

In c#6: "You are probably looking for a change made after 418694 (known good), but no later than 418695 (first known bad)." Which is the same as in c#2. 418695 points at cbiesinger@ CL.
What does it mean?

In general, CC animations are unlikely involved. "Even refreshing page doesn't help" - refresh re-creates the renderer process.