Disable flaky TabSearchInteractiveUITest on Mac.

Test sometimes fail on MacOS 10.15 builder. See e.g.:

https://ci.chromium.org/p/chromium/builders/ci/Mac10.15%20Tests/5298

TBR=romanarora@chromium.org

Bug: 1143663
Change-Id: I72a41ca282d02302f33d626544ba7980d6c356d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505988Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Roman Arora <romanarora@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822272}

chrome/test/data/webui/tab_search_merge/tab_search_interactive_ui_tests.js

@@ -34,6 +34,11 @@ var TabSearchInteractiveUITest = class extends PolymerInteractiveUITest {
   }
 };
 
-TEST_F('TabSearchInteractiveUITest', 'All', function() {
+GEN('#if defined(OS_MAC)');
+GEN('#define MAYBE_All DISABLED_All');
+GEN('#else');
+GEN('#define MAYBE_All All');
+GEN('#endif');
+TEST_F('TabSearchInteractiveUITest', 'MAYBE_All', function() {
   mocha.run();
 });