Re-enable TabletmodeTransitionTest

This has been disabled due to flakiness. The flakiness was
observed on asan bot and win7 bot. Now this test is only
built on non-sanitizer builds for chromeos (see r749747),
so it's safe to re-enable it.

As I tried running, it still fails sometimes on debug builds
on waiting logic, so I modified it slightly.

Bug: 1057864, 1057868
Test: interactive_ui_tests --gtest_repeat=20
Change-Id: I9e8112ffbd6f30f042ffd03f6b29bc7e1dbe18b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2103521Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750605}

ash/shell_test_api.cc

@@ -94,7 +94,6 @@ class WindowAnimationWaiter : public ui::LayerAnimationObserver {
       ui::LayerAnimationSequence* sequence) override {}
 
   void Wait() {
-    DCHECK(animator_->is_animating());
     run_loop_.Run();
   }
 

chrome/browser/ui/ash/tablet_mode_transition_interactive_uitest.cc

@@ -61,19 +61,22 @@ class TabletModeTransitionTest : public UIPerformanceTest {
   DISALLOW_COPY_AND_ASSIGN(TabletModeTransitionTest);
 };
 
-// TODO(http://crbug.com/1057864): Disabled due to flakiness.
-IN_PROC_BROWSER_TEST_F(TabletModeTransitionTest, DISABLED_EnterExit) {
+IN_PROC_BROWSER_TEST_F(TabletModeTransitionTest, EnterExit) {
   // Activate the first window. The top window is the only window which animates
   // and is the one we should check to see if the tablet animation has finished.
   Browser* browser = BrowserList::GetInstance()->GetLastActive();
   aura::Window* browser_window = browser->window()->GetNativeWindow();
+  ash::ShellTestApi shell_test_api;
 
-  ash::ShellTestApi().SetTabletModeEnabledForTest(
-      true, /*wait_for_completion=*/false);
-  ash::ShellTestApi().WaitForWindowFinishAnimating(browser_window);
+  auto waiter =
+      shell_test_api.CreateWaiterForFinishingWindowAnimation(browser_window);
+  shell_test_api.SetTabletModeEnabledForTest(true,
+                                             /*wait_for_completion=*/false);
+  std::move(waiter).Run();
 
-  ash::ShellTestApi().SetTabletModeEnabledForTest(
-      false, /*wait_for_completion=*/false);
-  EXPECT_TRUE(browser_window->layer()->GetAnimator()->is_animating());
-  ash::ShellTestApi().WaitForWindowFinishAnimating(browser_window);
+  waiter =
+      shell_test_api.CreateWaiterForFinishingWindowAnimation(browser_window);
+  shell_test_api.SetTabletModeEnabledForTest(false,
+                                             /*wait_for_completion=*/false);
+  std::move(waiter).Run();
 }