Also failed here on Android:
https://ci.chromium.org/buildbot/tryserver.chromium.android/android_n5x_swarming_rel/367248

I  460.587s run_tests_on_device(00c686290f5e32d3)  [ RUN      ] AccessibilityFullscreenBrowserTest.InsideIFrame
I  460.587s run_tests_on_device(00c686290f5e32d3)  [WARNING:dns_config_service_posix.cc(351)] Failed to read DnsConfig.
I  460.587s run_tests_on_device(00c686290f5e32d3)  [ERROR:devtools_http_handler.cc(281)] Cannot start http server for devtools. Stop devtools.
I  460.587s run_tests_on_device(00c686290f5e32d3)  [WARNING:child_process_launcher_helper_posix.cc(118)] Ignoring invalid file assets/snapshot_blob_32.bin
I  460.587s run_tests_on_device(00c686290f5e32d3)  [WARNING:simple_synchronous_entry.cc(1255)] Could not open platform files for entry.
I  460.587s run_tests_on_device(00c686290f5e32d3)  referenceTable head length=54 1
I  460.587s run_tests_on_device(00c686290f5e32d3)  ../../content/browser/accessibility/fullscreen_browsertest.cc:141: Failure
I  460.587s run_tests_on_device(00c686290f5e32d3)  Expected equality of these values:
I  460.587s run_tests_on_device(00c686290f5e32d3)    2
I  460.587s run_tests_on_device(00c686290f5e32d3)    CountLinks(manager->GetRoot())
I  460.587s run_tests_on_device(00c686290f5e32d3)      Which is: 1
I  460.587s run_tests_on_device(00c686290f5e32d3)  [  FAILED  ] AccessibilityFullscreenBrowserTest.InsideIFrame, where TypeParam =  and GetParam() =  (793 ms)

foolip@, can you PTAL?  It seems to me that your r533391 has introduced a flaky test (I see that it also flakes on https://ci.chromium.org/buildbot/chromium.fyi/Site%20Isolation%20Android/?limit=200).

FWIW, it is probably too late to revert.  It is a pity that back when opening this bug we didn't trace the problem to r533391 (which added content/test/data/accessibility/fullscreen/iframe.html).

Looking at the test code I can't spot anything obviously wrong, but apparently either 1) the test doesn't wait long enough or 2) the product code doesn't detect the new links in some cases.  Some notes / questions:

document.onwebkitfullscreenchange = function() {
  const iframe = document.createElement('iframe');
  iframe.srcdoc = '<a href="/">link inside iframe</a>';
  document.body.appendChild(iframe);
  iframe.onload = function() {
    document.webkitExitFullscreen();
    document.onwebkitfullscreenchange = function() {  <- why add event listener *after* triggering an event?
      button.setAttribute('aria-label', 'Done');  <- are all "link" ax nodes guaranteed to be known to the browser at this point?
    }
  }
}

Also on win10_chromium_x64_rel_ng:
https://ci.chromium.org/buildbot/tryserver.chromium.win/win10_chromium_x64_rel_ng/98905

Issue 821701 has been merged into this issue.

Marking a11y-secondary since this appears to be automation related. Please leave a comment if this is actually Windows UI based.