Web UI: Re-enable CrElementsInputV3Test on non-Mac

This test is flaky only on Mac, just like CrElementsInputTest. Re-enable
it on other platforms and add a comment referencing the bug for
CrElementsInputTest. The flakes are likely due to the same cause, since
these tests are identical other than the version of Polymer.

Bug: 997944
Change-Id: Ife5e77c066496f702a3661457aa68d0f2af808c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1785624Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693869}

chrome/test/data/webui/cr_elements/cr_elements_v3_focus_test.js

@@ -60,7 +60,13 @@ var CrElementsInputV3Test = class extends CrElementsV3FocusTest {
   }
 };
 
-TEST_F('CrElementsInputV3Test', 'DISABLED_All', function() {
+// https://crbug.com/997943: 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('CrElementsInputV3Test', 'MAYBE_All', function() {
   mocha.run();
 });