chromeos: disable CrExtensionsPackDialogTest.PackSuccess

It's flaky on ChromeOS as well.

BUG=877109
TEST=test only change
TBR=aee@chromium.org

Change-Id: Ibd12f914131ddeef465ab5c1b802c2cc0cb94aba
Reviewed-on: https://chromium-review.googlesource.com/c/1294089Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601678}

chrome/test/data/webui/extensions/cr_extensions_browsertest.js

@@ -526,7 +526,9 @@ TEST_F('CrExtensionsPackDialogTest', 'Interaction', function() {
 // http://crbug.com/832885
 // Temporarily disabling on Mac due to flaky dialog visibility failure.
 // http://crbug.com/877109
-GEN('#if defined(OS_WIN) || defined(OS_MACOSX)');
+// And flay on ChromeOS as well. Added comments about ChromeOS to
+// http://crbug.com/877109
+GEN('#if defined(OS_WIN) || defined(OS_MACOSX) || defined(OS_CHROMEOS)');
 GEN('#define MAYBE_PackSuccess DISABLED_PackSuccess');
 GEN('#else');
 GEN('#define MAYBE_PackSuccess PackSuccess');