Fix the issue that AppList shows the rounded corners in fullscreen

AppListView::OnWindowBoundsChanged fills the corners by translating
AppListBackgroundShield when AppList shows in HALF state and the virtual
keyboard is enabled. However, AppListView::Layout may be called after
AppListView::OnWindowBoundsChanged (the side effect of crrev.com/c/1547051)
and it resets the view's transform. As a result. the rounded corners
show again. Fix this issue by modifying
AppListView::UpdateAppListBackgroundYPosition which is
always called by AppListView::Layout.

Bug: 968698
Change-Id: I7a0852115ea4363c44a51e299af93fc6bc1c4623
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1656161
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668554}

ash/app_list/views/app_list_view.cc

@@ -2124,7 +2124,13 @@ void AppListView::UpdateAppListBackgroundYPosition() {
       transform.Translate(
           0, -kAppListBackgroundRadius * (app_list_transition_progress - 1));
     }
-  } else if (is_fullscreen()) {
+  } else if (is_fullscreen() ||
+             (app_list_state_ == ash::AppListViewState::kHalf &&
+              GetBoundsInScreen().y() == 0)) {
+    // AppListView::Layout may be called after OnWindowBoundsChanged. It may
+    // reset the transform of |app_list_background_shield_|. So hide the rounded
+    // corners when AppListView is in Half state and its bottom is on the
+    // display edge.
     transform.Translate(0, -kAppListBackgroundRadius);
   }
   app_list_background_shield_->SetTransform(transform);