[Aria tests] Disable a flaky rule (AX_ARIA_08)

See bug for many failing tests.
Example: https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests/builds/47887/steps/browser_tests%20on%20Ubuntu-12.04/logs/EditDictionaryWebUITest.testNoCloseOnSearchEnter

BUG=657514
TBR=aboxhall
NOTRY=true

Review-Url: https://chromiumcodereview.appspot.com/2436063002
Cr-Commit-Position: refs/heads/master@{#426514}

chrome/test/data/webui/test_api.js

@@ -211,6 +211,9 @@ var testing = {};
             // TODO(apacible): re-enable when following issue is fixed.
             // github.com/GoogleChrome/accessibility-developer-tools/issues/251
             "tableHasAppropriateHeaders",
+
+            // TODO(crbug.com/657514): This rule is flaky on Linux/ChromeOS.
+            "requiredOwnedAriaRoleMissing",
         ];
       }
       return this.accessibilityAuditConfig_;