slow: disable one history metrics test

This change disables GoogleSearchDomainMixingMetricsEmitterTest.
EmitsMetricsOnStart, which is the slowest unit test in the codebase by
a factor of >2x. This test runs slowly because it simulates an entire
month worth of history expiration, which amounts to running the same
no-op every 5 simulated "minutes" hundreds of thousands of times.

Since this code is all slated to be deleted soon (per the linked
bug), the test is just disabled.

Bug: 1040458
Change-Id: Idfc0115d00cf9c3b763c07b5715b3b39e5b45c5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057445
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741556}

chrome/browser/google/google_search_domain_mixing_metrics_emitter_unittest.cc

@@ -136,7 +136,11 @@ TEST_F(GoogleSearchDomainMixingMetricsEmitterTest, WaitsUntilNeeded) {
             task_environment_.NextMainThreadPendingTaskDelay());
 }
 
-TEST_F(GoogleSearchDomainMixingMetricsEmitterTest, EmitsMetricsOnStart) {
+// Disabled pending deletion, see https://crbug.com/1040458
+// This test takes several seconds of CPU time to run because it simulates a
+// month worth of history expiration running, which happens every 300 seconds.
+TEST_F(GoogleSearchDomainMixingMetricsEmitterTest,
+       DISABLED_EmitsMetricsOnStart) {
   // Metrics were computed up to 4am on Jan 1st.
   base::Time last_metrics_time;
   ASSERT_TRUE(