Disable DesktopEngagementServiceTest.TestTimeoutDiscount on Mac.

chrome/browser/metrics/desktop_engagement/desktop_engagement_service_unittest.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/metrics/desktop_engagement/desktop_engagement_service.h"
 
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/test/histogram_tester.h"
 #include "base/test/test_mock_time_task_runner.h"
@@ skipping 130 matching lines @@
   while (!instance.is_timeout()) {
     base::RunLoop().RunUntilIdle();
   }
 
   EXPECT_FALSE(instance.in_session());
   EXPECT_FALSE(instance.is_visible());
   EXPECT_FALSE(instance.is_audio_playing());
   histogram_tester.ExpectTotalCount("Session.TotalDuration", 1);
 }
 
-TEST(DesktopEngagementServiceTest, TestTimeoutDiscount) {
+// Flaky on Mac Debug. See http://crbug.com/646758
+#if defined(OS_MACOSX)
+#define MAYBE_TestTimeoutDiscount DISABLED_TestTimeoutDiscount
+#else
+#define MAYBE_TestTimeoutDiscount TestTimeoutDiscount
+#endif
+TEST(DesktopEngagementServiceTest, MAYBE_TestTimeoutDiscount) {
   base::MessageLoop loop(base::MessageLoop::TYPE_DEFAULT);
   base::HistogramTester histogram_tester;
   MockDesktopEngagementService instance;
 
   int inactivity_interval = 2;
   instance.SetInactivityTimeoutForTesting(inactivity_interval);
 
   instance.OnVisibilityChanged(true);
   base::TimeTicks before_session_start = base::TimeTicks::Now();
   instance.OnUserEvent(); // This should start the session
   histogram_tester.ExpectTotalCount("Session.TotalDuration", 0);
 
   // Wait until the session expires.
   while (!instance.is_timeout()) {
     base::RunLoop().RunUntilIdle();
   }
   base::TimeTicks after_session_end = base::TimeTicks::Now();
 
   histogram_tester.ExpectTotalCount("Session.TotalDuration", 1);
   // The recorded value should be shorter than the specified inactivity
   // interval.
   base::Bucket bucket =
       histogram_tester.GetAllSamples("Session.TotalDuration")[0];
   EXPECT_LE(
     bucket.min + inactivity_interval,
     (after_session_end - before_session_start).InSeconds());
 }