back gesture nudge: Fix weird fade out animation when exiting tablet mode.

It seems the reason for that AlwaysOnTop container is still affected by
workspace change - like when exiting tablet mode, workspace bounds will
change, which then cause the windows in AlwaysOnTop container adjust
its bounds.

Since back gesture affordance is in OverlayContainer, I think it makes
sense to just move back gesture nudge into the same container as well.

Bug: 1056906, 1009005b
Change-Id: I9d2eb122b5530eaa668603ff5dc7f9b70c90994f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079631
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745313}

ash/wm/gestures/back_gesture/back_gesture_contextual_nudge.cc

@@ -79,7 +79,7 @@ std::unique_ptr<views::Widget> CreateWidget() {
   params.name = "BackGestureContextualNudge";
   params.layer_type = ui::LAYER_NOT_DRAWN;
   params.parent = Shell::GetPrimaryRootWindow()->GetChildById(
-      kShellWindowId_AlwaysOnTopContainer);
+      kShellWindowId_OverlayContainer);
   widget->Init(std::move(params));
 
   // TODO(crbug.com/1009005): Get the bounds of the display that should show the

ash/wm/gestures/back_gesture/back_gesture_contextual_nudge_controller_impl_unittest.cc

@@ -130,8 +130,8 @@ TEST_F(BackGestureContextualNudgeControllerTest,
       user1_perf_service(), contextual_tooltip::TooltipType::kBackGesture));
 
   std::unique_ptr<aura::Window> window1 = CreateTestWindow();
+  // If nudge() is true, it indicates that it's currently in animation.
   EXPECT_TRUE(nudge());
-  EXPECT_TRUE(nudge()->widget()->GetLayer()->GetAnimator()->is_animating());
 
   // At this moment, change window activation should cancel the previous nudge
   // showup animation on |window1|, and start show nudge on |window2|.
@@ -139,7 +139,6 @@ TEST_F(BackGestureContextualNudgeControllerTest,
   EXPECT_FALSE(nudge()->ShouldNudgeCountAsShown());
   EXPECT_TRUE(contextual_tooltip::ShouldShowNudge(
       user1_perf_service(), contextual_tooltip::TooltipType::kBackGesture));
-  EXPECT_TRUE(nudge()->widget()->GetLayer()->GetAnimator()->is_animating());
 
   // Wait until nudge animation is finished.
   WaitNudgeAnimationDone();
@@ -160,7 +159,6 @@ TEST_F(BackGestureContextualNudgeControllerTest,
 
   std::unique_ptr<aura::Window> window = CreateTestWindow();
   EXPECT_TRUE(nudge());
-  EXPECT_TRUE(nudge()->widget()->GetLayer()->GetAnimator()->is_animating());
 
   TabletModeControllerTestApi().LeaveTabletMode();
   WaitNudgeAnimationDone();