Disable OSSettingsSmbTest.All for debug builds

This test is flaky on debug builds; mostly fails with timeouts.
For now, it's better to disable this test case on debug builds.
This is somewhat a common pattern among webui tests -- and I think
it's fine as long as it's tested on the release builds.

Bug: 968608
Test: none
Change-Id: I09b19f1e1854f9e5943b55b75e61abef777c493e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639143Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Jordy Greenblatt <jordynass@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665309}

chrome/test/data/webui/settings/chromeos/os_settings_browsertest.js

@@ -59,6 +59,13 @@ var OSSettingsSmbPageTest = class extends OSSettingsBrowserTest {
   }
 };
 
-TEST_F('OSSettingsSmbPageTest', 'All', function() {
+// OSSettingsSmbPageTest.All is flaky on debug. See https://crbug.com/968608.
+GEN('#if !defined(NDEBUG)');
+GEN('#define MAYBE_All DISABLED_All');
+GEN('#else');
+GEN('#define MAYBE_All All');
+GEN('#endif');
+
+TEST_F('OSSettingsSmbPageTest', 'MAYBE_All', function() {
   mocha.run();
 });