CrOS: Fix appearance of notification toasts when sliding out via gesture

ui/message_center/BUILD.gn

Index: ui/message_center/BUILD.gn
diff --git a/ui/message_center/BUILD.gn b/ui/message_center/BUILD.gn
index 08aff8c3232f36979f25ab0e37067df2a4302a57..95686459a125f397ba21a62da66ce59bc02b9ffb 100644
--- a/ui/message_center/BUILD.gn
+++ b/ui/message_center/BUILD.gn
@@ -150,6 +150,8 @@ component("message_center") {
         "views/popup_alignment_delegate.h",
         "views/proportional_image_view.cc",
         "views/proportional_image_view.h",
+        "views/slide_out_controller.cc",
+        "views/slide_out_controller.h",
         "views/toast_contents_view.cc",
         "views/toast_contents_view.h",
       ]
@@ -221,6 +223,7 @@ test("message_center_unittests") {
     "//ui/base",
     "//ui/base:test_support",
     "//ui/events",
+    "//ui/events:test_support",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/gl",