Start home fade in from overview sooner

Makes home scale and fade in animation run in parallel with overview
fadeout (instead of waiting for the overview animation to end).

Change-Id: Ica9bd3cc98faf5dd2827fe6871a1f4b7b5d7c953
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918664Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715826}

ash/home_screen/home_screen_controller.cc

@@ -267,19 +267,38 @@ void HomeScreenController::OnOverviewModeStarting() {
 
 void HomeScreenController::OnOverviewModeEnding(
     OverviewSession* overview_session) {
-  // The launcher should be shown after overview mode finishes animating, in
-  // OnOverviewModeEndingAnimationComplete(). Overview however is nullptr by the
-  // time the animations are finished, so cache the exit type here.
+  // The launcher will be shown after overview mode finishes animating, in
+  // OnOverviewModeEndingAnimationComplete(). Overview however is nullptr by
+  // the time the animations are finished, so cache the exit type here.
   overview_exit_type_ =
       base::make_optional(overview_session->enter_exit_overview_type());
+
+  // If the overview is fading out, start the home screen animation in parallel.
+  // Otherwise the transition will be initiated in
+  // OnOverviewModeEndingAnimationComplete().
+  if (overview_session->enter_exit_overview_type() ==
+      OverviewSession::EnterExitOverviewType::kFadeOutExit) {
+    home_screen_presenter_.ScheduleOverviewModeAnimation(
+        HomeScreenPresenter::TransitionType::kScaleHomeIn, true /*animate*/);
+
+    // Make sure the window visibility is updated, in case it was previously
+    // hidden due to overview being shown.
+    UpdateVisibility();
+  }
 }
 
 void HomeScreenController::OnOverviewModeEndingAnimationComplete(
     bool canceled) {
-  if (canceled)
-    return;
   DCHECK(overview_exit_type_.has_value());
 
+  // For kFadeOutExit EnterExitOverviewType, the home animation is scheduled in
+  // OnOverviewModeEnding(), so there is nothing else to do at this point.
+  if (canceled || *overview_exit_type_ ==
+                      OverviewSession::EnterExitOverviewType::kFadeOutExit) {
+    overview_exit_type_ = base::nullopt;
+    return;
+  }
+
   const bool animate =
       *overview_exit_type_ ==
           OverviewSession::EnterExitOverviewType::kSlideOutExit ||