Deflake AddFirstWallpaperAnimationEndCallback test

This test is currently flaky because it relies upon the ordering
of delayed and non delayed tasks to quit a run loop before the
very short (0.01s) animation delay is up. This patch deflakes it by
temporarily freezing time to prevent the animation from
completing before the test expects it to.

TBR=wzang@chromium.org

Bug: 863341, 891670, 910493
Change-Id: I34cf2824c387c380134eb38b3375e6ab79de4be8
Reviewed-on: https://chromium-review.googlesource.com/c/1365244
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614322}

ash/wallpaper/wallpaper_controller_unittest.cc

@@ -29,6 +29,7 @@
 #include "base/task/post_task.h"
 #include "base/task/task_scheduler/task_scheduler.h"
 #include "base/test/bind_test_util.h"
+#include "base/time/time_override.h"
 #include "chromeos/chromeos_switches.h"
 #include "components/prefs/testing_pref_service.h"
 #include "mojo/public/cpp/bindings/associated_binding.h"
@@ -206,6 +207,7 @@ class TaskObserver : public base::MessageLoop::TaskObserver {
 
 // See content::RunAllTasksUntilIdle().
 void RunAllTasksUntilIdle() {
+  LOG(ERROR) << "RunAllTasksUntilIdle - before";
   while (true) {
     TaskObserver task_observer;
     base::MessageLoopCurrent::Get()->AddTaskObserver(&task_observer);
@@ -218,6 +220,7 @@ void RunAllTasksUntilIdle() {
     if (!task_observer.processed())
       break;
   }
+  LOG(ERROR) << "RunAllTasksUntilIdle - after";
 }
 
 // A test implementation of the WallpaperObserver mojo interface.
@@ -2334,9 +2337,26 @@ TEST_F(WallpaperControllerTest, AddFirstWallpaperAnimationEndCallback) {
       base::BindLambdaForTesting(
           [&is_second_callback_run]() { is_second_callback_run = true; }),
       test_window.get());
+  {
+    // The animation is quite short (0.01 seconds) which is problematic in
+    // debug builds if RunAllTasksUntilIdle is a bit slow to execute. That leads
+    // to test flakes. We work around that by temporarily freezing time, which
+    // prevents the animation from unexpectedly completing too soon.
+    // Ideally this test should use MockTime instead, which will become easier
+    // after https://crrev.com/c/1352260 lands.
+    base::subtle::ScopedTimeClockOverrides time_override(
+        nullptr,
+        []() {
+          static base::TimeTicks time_ticks =
+              base::subtle::TimeTicksNowIgnoringOverride();
+          return time_ticks;
+        },
+        nullptr);
+
+    RunAllTasksUntilIdle();
+  }
   // Neither callback is run because the animation of the first wallpaper
   // hasn't finished yet.
-  RunAllTasksUntilIdle();
   EXPECT_FALSE(is_first_callback_run);
   EXPECT_FALSE(is_second_callback_run);