Deflake AssistantOnboardingView unittest.

Previously, the unittest could attempt to advance the mock clock by a
negative amount which is disallowed by the framework.

Bug: 1092026
Change-Id: I47ce0ecc69b4391cff08154138c001bf7062f69a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2236214Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Jeroen Dhollander <jeroendh@google.com>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#776232}

ash/assistant/ui/main_stage/assistant_onboarding_view_unittest.cc

@@ -73,13 +73,9 @@ class AssistantOnboardingViewTest : public AshTestBase {
 // Tests -----------------------------------------------------------------------
 
 TEST_F(AssistantOnboardingViewTest, ShouldHaveExpectedGreeting) {
-  // Advance clock to midnight.
-  base::Time::Exploded now;
-  base::Time::Now().LocalExplode(&now);
-  AdvanceClock(base::TimeDelta::FromHours((24 - now.hour) % 24) -
-               base::TimeDelta::FromMinutes(now.minute) -
-               base::TimeDelta::FromSeconds(now.second) -
-               base::TimeDelta::FromMilliseconds(now.millisecond));
+  // Advance clock to midnight tomorrow.
+  AdvanceClock(base::Time::Now().LocalMidnight() +
+               base::TimeDelta::FromHours(24) - base::Time::Now());
 
   // Verify 4:59 AM.
   AdvanceClock(base::TimeDelta::FromHours(4) +