This seems to be flaky on today's tree.  Here's two builds where these errors are occurring:
https://build.chromium.org/p/chromium.win/builders/Win%207%20Tests%20x64%20%281%29/builds/13125
https://build.chromium.org/p/chromium.win/builders/Win%207%20Tests%20x64%20%281%29/builds/13144

The tests are:
SettingsFormatWebUITest.CheckboxIdOrPrefCheck
OptionsWebUITest.testSetBooleanPrefTriggers
OptionsWebUITest.EnableDoNotTrackAndCancelInterstitial
 

Example output:

SettingsFormatWebUITest.CheckboxIdOrPrefCheck (run #1):
[ RUN      ] SettingsFormatWebUITest.CheckboxIdOrPrefCheck
[4420:592:0425/130152:WARNING:CONSOLE(0)] "/deep/ combinator is deprecated. See https://www.chromestatus.com/features/6750456638341120 for more details.", source:  (0)
[4420:592:0425/130153:INFO:CONSOLE(1196)] "Running TestCase SettingsFormatWebUITest.CheckboxIdOrPrefCheck", source: test_api.js (1196)
[4420:592:0425/130153:ERROR:web_ui_test_handler.cc(76)] Failed: RUN_TEST_F("SettingsFormatWebUITest","CheckboxIdOrPrefCheck")
Error:
Accessibility issues found on chrome://settings-frame/
*** Begin accessibility audit results ***
An accessibility audit found
Errors:
Error: AX_ARIA_08 (Elements with ARIA roles must ensure required owned elements are present) failed on the following elements (1 - 2 of 2):
#default-search-engine-list
#other-search-engine-list
See https://github.com/GoogleChrome/accessibility-developer-tools/wiki/Audit-Rules#ax_aria_08 for more information.
*** End accessibility audit results ***
    at Object.runAccessibilityAudit (test_api.js:371:17)
    at Object.runAccessibilityAudit (test_api.js:501:22)
    at test_api.js:995:20
    at testDone (test_api.js:783:31)
    at runTest (test_api.js:1042:7)
    at <anonymous>:1:1
  Actual: false
Expected: true
[  FAILED  ] SettingsFormatWebUITest.CheckboxIdOrPrefCheck, where TypeParam =  and GetParam() =  (1730 ms)

Another failure here (on Win7 Tests (1)):
https://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%281%29/builds/51751

AdvancedSettingsWebUITest.testAdvancedSettingsShown
AutofillOptionsWebUITest.testOpenAutofillOptions

Same errors on #default-search-engine-list and #other-search-engine-list:

AdvancedSettingsWebUITest.testAdvancedSettingsShown (run #1):
[ RUN      ] AdvancedSettingsWebUITest.testAdvancedSettingsShown
[4644:2560:0425/150609:WARNING:CONSOLE(0)] "/deep/ combinator is deprecated. See https://www.chromestatus.com/features/6750456638341120 for more details.", source:  (0)
[4644:2560:0425/150610:INFO:CONSOLE(1196)] "Running TestCase AdvancedSettingsWebUITest.testAdvancedSettingsShown", source: test_api.js (1196)
[4644:2560:0425/150611:ERROR:web_ui_test_handler.cc(76)] Failed: RUN_TEST_F("AdvancedSettingsWebUITest","testAdvancedSettingsShown")
Error:
Accessibility issues found on chrome://settings-frame/autofill
*** Begin accessibility audit results ***
An accessibility audit found
Errors:
Error: AX_ARIA_08 (Elements with ARIA roles must ensure required owned elements are present) failed on the following elements (1 - 2 of 2):
#default-search-engine-list
#other-search-engine-list
See https://github.com/GoogleChrome/accessibility-developer-tools/wiki/Audit-Rules#ax_aria_08 for more information.


*** End accessibility audit results ***
    at Object.runAccessibilityAudit (test_api.js:371:17)
    at Object.runAccessibilityAudit (test_api.js:501:22)
    at test_api.js:995:20
    at testDone (test_api.js:783:31)
    at runTest (test_api.js:1042:7)
    at <anonymous>:1:1
  Actual: false
Expected: true
[  FAILED  ] AdvancedSettingsWebUITest.testAdvancedSettingsShown, where TypeParam =  and GetParam() =  (2681 ms)

Tracking these in issue 606657.