Disabling flaky CrExtensionsPackDialogTest.PackWarning test

TBR=aee@chromium.org
BUG=877109

Change-Id: Ie95d35f83e8016ae9b0c4b60f67481217a2e3c6d
Reviewed-on: https://chromium-review.googlesource.com/c/1292710Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601350}

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

@@ -539,7 +539,14 @@ TEST_F('CrExtensionsPackDialogTest', 'PackError', function() {
   this.runMochaTest(extension_pack_dialog_tests.TestNames.PackError);
 });
 
-TEST_F('CrExtensionsPackDialogTest', 'PackWarning', function() {
+// Temporarily disabling on Mac due to flakiness.
+// http://crbug.com/877109
+GEN('#if defined(OS_MACOSX)');
+GEN('#define MAYBE_PackWarning DISABLED_PackWarning');
+GEN('#else');
+GEN('#define MAYBE_PackWarning PackWarning');
+GEN('#endif');
+TEST_F('CrExtensionsPackDialogTest', 'MAYBE_PackWarning', function() {
   this.runMochaTest(extension_pack_dialog_tests.TestNames.PackWarning);
 });