Add ink drop masking to TrayBackgroundView

ash/common/system/tray/tray_background_view.cc

Index: ash/common/system/tray/tray_background_view.cc
diff --git a/ash/common/system/tray/tray_background_view.cc b/ash/common/system/tray/tray_background_view.cc
index cf52b94756457b0ce779e8acc491cf5f05e72c42..b2ad7299bec648a365af2bff3fbe018e9bc04043 100644
--- a/ash/common/system/tray/tray_background_view.cc
+++ b/ash/common/system/tray/tray_background_view.cc
@@ -35,6 +35,7 @@
 #include "ui/gfx/transform.h"
 #include "ui/views/animation/flood_fill_ink_drop_ripple.h"
 #include "ui/views/animation/ink_drop_highlight.h"
+#include "ui/views/animation/ink_drop_mask.h"
 #include "ui/views/background.h"
 #include "ui/views/layout/box_layout.h"
 #include "ui/wm/core/window_animations.h"
@@ -572,6 +573,14 @@ void TrayBackgroundView::SetSeparatorVisibility(bool is_shown) {
   SchedulePaint();
 }
 
+std::unique_ptr<views::InkDropMask> TrayBackgroundView::CreateInkDropMask()
+    const {
+  return base::MakeUnique<views::RoundRectInkDropMask>(
+      GetLocalBounds(),
+      GetBackgroundBounds(GetContentsBounds(), shelf_alignment_),
+      kTrayRoundedBorderRadius);
+}
+
 bool TrayBackgroundView::ShouldEnterPushedState(const ui::Event& event) {
   if (is_active_)
     return false;