Disable flaky GoogleSearchDomainMixingMetricsEmitterTest

Specifically, .EmitsMetricsWhenTimerFires.
I know these tests are going away soon, but I'll like to get
the failures off Sheriff-Omatic.

Bug: 1040458
Change-Id: Ib7601ccc2291a5e2813e5edacbac68f93d004d98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015484
Auto-Submit: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734180}

chrome/browser/google/google_search_domain_mixing_metrics_emitter_unittest.cc

@@ -162,7 +162,8 @@ TEST_F(GoogleSearchDomainMixingMetricsEmitterTest, EmitsMetricsOnStart) {
   VerifyHistograms(tester);
 }
 
-TEST_F(GoogleSearchDomainMixingMetricsEmitterTest, EmitsMetricsWhenTimerFires) {
+TEST_F(GoogleSearchDomainMixingMetricsEmitterTest,
+       DISABLED_EmitsMetricsWhenTimerFires) {
   // Metrics were computed up to 4am on Jan 1st.
   base::Time last_metrics_time;
   ASSERT_TRUE(