Disable flaky test on Mac.

Bug: 945846
TBR: noel
Change-Id: I9b62ee039e537cc6485514d7e744b6a702b2bb17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539212Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644602}

chrome/test/data/webui/print_preview/new_print_preview_ui_browsertest.js

@@ -189,7 +189,13 @@ PrintPreviewSettingsSelectTest = class extends NewPrintPreviewTest {
   }
 };
 
-TEST_F('PrintPreviewSettingsSelectTest', 'All', function() {
+// TODO(https://crbug.com/945846): Flaky on Mac.
+GEN('#if defined(OS_MACOSX)');
+GEN('#define MAYBE_All DISABLED_All');
+GEN('#else');
+GEN('#define MAYBE_All All');
+GEN('#endif');
+TEST_F('PrintPreviewSettingsSelectTest', 'MAYBE_All', function() {
   mocha.run();
 });