What steps will reproduce the problem?

(1) Go to http://csreis.github.io/tests/cross-site-iframe-simple.html
(2) Ctrl-F and search for "bridge".  This should highlight two matches in the iframe (which will be an OOPIF on desktop platforms)
(3) Hit backspace and type "x" to change the query to "bridgx"
(4) Hit backspace and type "e" to change query back to "bridge"

What is the expected result?
Two matches for "bridge" are highlighted once again, match counter shows "1/2", and ctrl-g alternates between the two matches.

What happens instead?
No matches are highlighted, find-in-page match counter shows "0/1", and going to the next match doesn't do anything.

Also, step (2) appears to not work right away sometimes.  If I hit ctrl-g a few times or click on the page to change focus, it typically starts working.

I reproed this on Linux and CrOS M70 as well as Linux ToT.  Coudln't repro on Windows.  creis@ also observed similar behavior on an internal b/ page involving same-origin iframes.

I did a bisect and got the following:
https://chromium.googlesource.com/chromium/src/+log/04bab18044ca5689ac72a05883051d23b9f354df..f39cb55a1451df9a1f48fb2f27a0789db2f74428

Of these, r584034 seems most likely since it touches find in page.  rakina@, can you please take a look?