Fix the issue that ScrollableShelf has no gradient zone

Layer::Clone(), which may be triggered by screen rotation, does not
copy the mask layer. As a result, gradient zone for scrollable
shelf may not show after screen rotation. This CL fixes the issue
by checking the mask layer in ScrollableShelfView::Layout() and
setting the mask layer if necessary.

Bug: 1035715
Change-Id: I0f8adb1df9991066c2e6ff46a401a561fdb15204
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986055
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728326}

ash/shelf/scrollable_shelf_view.cc

@@ -733,6 +733,13 @@ void ScrollableShelfView::Layout() {
   if (right_arrow_->GetVisible())
     right_arrow_->SetBoundsRect(right_arrow_bounds);
 
+  // Layer::Clone(), which may be triggered by screen rotation, does not copy
+  // the mask layer. So we may need to reset the mask layer.
+  if (!layer()->layer_mask_layer()) {
+    DCHECK(!gradient_layer_delegate_->layer()->layer_mask_back_link());
+    layer()->SetMaskLayer(gradient_layer_delegate_->layer());
+  }
+
   if (gradient_layer_delegate_->layer()->bounds() != layer()->bounds())
     gradient_layer_delegate_->layer()->SetBounds(layer()->bounds());
 
@@ -898,6 +905,11 @@ void ScrollableShelfView::ScrollRectToVisible(const gfx::Rect& rect) {
   ScrollToMainOffset(main_axis_offset_after_scroll, /*animating=*/true);
 }
 
+std::unique_ptr<ui::Layer> ScrollableShelfView::RecreateLayer() {
+  layer()->SetMaskLayer(nullptr);
+  return views::View::RecreateLayer();
+}
+
 const char* ScrollableShelfView::GetClassName() const {
   return "ScrollableShelfView";
 }

ash/shelf/scrollable_shelf_view.h

@@ -177,6 +177,7 @@ class ASH_EXPORT ScrollableShelfView : public views::AccessiblePaneView,
   void ViewHierarchyChanged(
       const views::ViewHierarchyChangedDetails& details) override;
   void ScrollRectToVisible(const gfx::Rect& rect) override;
+  std::unique_ptr<ui::Layer> RecreateLayer() override;
 
   // ShelfButtonDelegate:
   void OnShelfButtonAboutToRequestFocusFromTabTraversal(ShelfButton* button,