[Sheriff] Disable PageAdDensityIgnoreDisplayNoneFrame (flakes)

This test has been failing intermittently recently. Flake Portal link:
https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyawsSBUZsYWtlImBjaHJvbWl1bUBicm93c2VyX3Rlc3RzQEFkc1BhZ2VMb2FkTWV0cmljc09ic2VydmVyQnJvd3NlclRlc3QuUGFnZUFkRGVuc2l0eUlnbm9yZURpc3BsYXlOb25lRnJhbWUM

It's already been disabled on Windows, but it's been failing on other
platforms, too.

I've added a few labels to the linked bug to help track.

R=johnidel

Bug: 1121444
Change-Id: I5ab328a6967437203c6390f71e6fe13f5c24eb81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631192
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Reviewed-by: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843870}

chrome/browser/page_load_metrics/observers/ad_metrics/ads_page_load_metrics_observer_browsertest.cc

@@ -494,15 +494,11 @@ IN_PROC_BROWSER_TEST_F(AdsPageLoadMetricsObserverBrowserTest,
 
 // Creates a frame with display:none styling and verifies that it has an
 // empty intersection with the main frame.
-#if defined(OS_WIN)
-#define MAYBE_PageAdDensityIgnoreDisplayNoneFrame \
-  DISABLED_PageAdDensityIgnoreDisplayNoneFrame
-#else
-#define MAYBE_PageAdDensityIgnoreDisplayNoneFrame \
-  PageAdDensityIgnoreDisplayNoneFrame
-#endif
+//
+// TODO(crbug.com/1121444): This test is disabled due to flaky failures on
+// multiple platforms.
 IN_PROC_BROWSER_TEST_F(AdsPageLoadMetricsObserverBrowserTest,
-                       MAYBE_PageAdDensityIgnoreDisplayNoneFrame) {
+                       DISABLED_PageAdDensityIgnoreDisplayNoneFrame) {
   base::HistogramTester histogram_tester;
   ukm::TestAutoSetUkmRecorder ukm_recorder;
   auto waiter = CreatePageLoadMetricsTestWaiter();