[Trivial] Explanatory sheriff comment for AddSupervisionBrowserTest.UMATest

Simple follow up CL to add a comment and link to bug that was mistakenly
omitted during sheriffing.

See: https://chromium-review.googlesource.com/c/chromium/src/+/1880408

TBR=tobyhuang@chromium.org

Change-Id: I4d63454c6e9f960907bfa45a05cdc92fe54f5f6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891812Reviewed-by: Meredith Lane <meredithl@chromium.org>
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711623}

chrome/browser/ui/webui/chromeos/add_supervision/add_supervision_ui_browsertest.cc

@@ -204,6 +204,8 @@ IN_PROC_BROWSER_TEST_F(AddSupervisionBrowserTest,
   ASSERT_TRUE(ConfirmSignoutDialog::IsShowing());
 }
 
+// Disabled on ASan and LSAn builds, because it's very flaky. See
+// crbug.com/1004237
 #if defined(ADDRESS_SANITIZER) || defined(LEAK_SANITIZER)
 #define MAYBE_UMATest DISABLED_UMATest
 #else