Chrome Version: r513797
OS: macOS 10.12.6 (also repros on 10.13)

What steps will reproduce the problem?
(1) Spin up a test server that requests a client certificate. This should do the trick:
    cd third_party/boringssl/src/ssl/test/runner
    openssl s_server -cert cert.pem -key key.pem -www -accept 4433 -Verify 1
(2) Go to https://localhost:4433 in Chrome.
(3) If no client certificate dialog comes up, here is a really silly way to add one to Keychain. (But I think if you're on a corp Mac, you'll have the dialog come up without doing this?)
    a) Go to https://davidben.scripts.mit.edu/keygen_webcrypto/
    b) Hit the first "Get certificate" button.
    c) Hit the "Download certificate" link.
    d) Save and open the downloaded p12 file. Type 123 in the password prompt.
    Follow those steps and repeat step (2).

What is the expected result?
The client certificate prompt is responsive.

What happens instead?
The UI is frozen.

I did a bisect and it gave:
You are probably looking for a change made after 513788 (known good), but no later than 513797 (first known bad).
CHANGELOG URL:
https://chromium.googlesource.com/chromium/src/+log/51af90bc8f9ed49daa83872bb04a74cc89c8b842..bf6c42efe66fa71309a794abcb34a50e9713df74

This CL looks like the only change that plausibly changed Mac UI? Tentatively assigning to sdy@
https://chromium.googlesource.com/chromium/src/+/8f1989aa6b1becf26cafbcd3dc972a43009d3883

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