tablet: Reenable TabletModeControllerScreenshot*.ScreenshotVisibility.

Re-try of crrev.com/c/2182409. The change is the same but this time I
was able to locally repro the original failure (and have no failures
running this) and some fixes have landed since the last attempt.

Test: none
Bug: 1035356
Change-Id: Idc53b58a70f68be05b7ad36efb18c6facdb3ad56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2226784Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774413}

ash/wm/tablet_mode/tablet_mode_controller_unittest.cc

@@ -1776,10 +1776,7 @@ TEST_P(TabletModeControllerScreenshotTest, EnterTabletModeWhileAnimating) {
 
 // Tests that the screenshot is visible when a window animation happens when
 // entering tablet mode.
-// TODO(http://crbug.com/1035356): This test fails on bots but not locally so
-// suspected to be a timing issue. Possible that the screenshot is deleted
-// before the waiter is done waiting.
-TEST_P(TabletModeControllerScreenshotTest, DISABLED_ScreenshotVisibility) {
+TEST_P(TabletModeControllerScreenshotTest, ScreenshotVisibility) {
   auto window = CreateTestWindow(gfx::Rect(200, 200));
   auto window2 = CreateTestWindow(gfx::Rect(300, 200));
 
@@ -1793,6 +1790,13 @@ TEST_P(TabletModeControllerScreenshotTest, DISABLED_ScreenshotVisibility) {
   EXPECT_FALSE(IsScreenshotShown());
   EXPECT_FALSE(IsShelfOpaque());
 
+  EXPECT_FALSE(window2->layer()->GetAnimator()->is_animating());
+  // The layer we observer is actually the windows layer before starting the
+  // animation. The animation performed is a cross-fade animation which
+  // copies the window layer to another layer host. So cache them here for
+  // later use.
+  ui::Layer* old_layer = window2->layer();
+
   // Tests that after waiting for the async tablet mode entry, the screenshot is
   // shown.
   waiter.Wait();
@@ -1801,6 +1805,7 @@ TEST_P(TabletModeControllerScreenshotTest, DISABLED_ScreenshotVisibility) {
   EXPECT_TRUE(IsShelfOpaque());
 
   // Tests that the screenshot is destroyed after the window is done animating.
+  old_layer->GetAnimator()->StopAnimating();
   window2->layer()->GetAnimator()->StopAnimating();
   EXPECT_FALSE(IsScreenshotShown());
   EXPECT_TRUE(IsShelfOpaque());