overview: Never animate wallpaper when app dragging.

Test: manual
Bug: 936282
Change-Id: Ie9202b3f997e5c3b7d6214958b6d39eb4ffce412
Reviewed-on: https://chromium-review.googlesource.com/c/1493086Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636300}

ash/wm/overview/overview_grid.cc

@@ -976,6 +976,12 @@ void OverviewGrid::OnStartingAnimationComplete() {
 }
 
 bool OverviewGrid::ShouldAnimateWallpaper() const {
+  // Never animate when doing app dragging.
+  if (overview_session_->enter_exit_overview_type() ==
+      OverviewSession::EnterExitOverviewType::kWindowDragged) {
+    return false;
+  }
+
   // If one of the windows covers the workspace, we do not need to animate.
   for (const auto& overview_item : window_list_) {
     if (CanCoverAvailableWorkspace(overview_item->GetWindow()))