Disable DumpAccessibilityTreeTest.AccessibilityIframeCrossProcess

This test is flaky.

Dashboard:
https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyYgsSBUZsYWtlIldjaHJvbWl1bUBjb250ZW50X2Jyb3dzZXJ0ZXN0c0BEdW1wQWNjZXNzaWJpbGl0eVRyZWVUZXN0LkFjY2Vzc2liaWxpdHlJZnJhbWVDcm9zc1Byb2Nlc3MM

Bug: 622387
Change-Id: I7390a0250b4005c0361788a592f4fbecf1ff4ee1
Tbr: dmazzoni@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144402Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757980}

content/browser/accessibility/dump_accessibility_tree_browsertest.cc

@@ -1577,8 +1577,9 @@ IN_PROC_BROWSER_TEST_P(DumpAccessibilityTreeTest,
   RunHtmlTest(FILE_PATH_LITERAL("iframe.html"));
 }
 
+// https://crbug.com/622387
 IN_PROC_BROWSER_TEST_P(DumpAccessibilityTreeTest,
-                       AccessibilityIframeCrossProcess) {
+                       DISABLED_AccessibilityIframeCrossProcess) {
   RunHtmlTest(FILE_PATH_LITERAL("iframe-cross-process.html"));
 }