Disable flaky DumpAccessibilityTreeTest.AccessibilityIframeCoordinates

Fails on content_browsertests about once every 4 cycles. See bug.

BUG=224659
TBR=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/905203002

Cr-Commit-Position: refs/heads/master@{#315252}

content/browser/accessibility/dump_accessibility_tree_browsertest.cc

@@ -745,8 +745,9 @@ IN_PROC_BROWSER_TEST_F(DumpAccessibilityTreeTest, AccessibilityIframe) {
   RunHtmlTest(FILE_PATH_LITERAL("iframe.html"));
 }
 
+// Flaky. See http://crbug.com/224659.
 IN_PROC_BROWSER_TEST_F(DumpAccessibilityTreeTest,
-                       AccessibilityIframeCoordinates) {
+                       DISABLED_AccessibilityIframeCoordinates) {
   RunHtmlTest(FILE_PATH_LITERAL("iframe-coordinates.html"));
 }