Add a floating close button for arc custom notification

ui/arc/BUILD.gn

Index: ui/arc/BUILD.gn
diff --git a/ui/arc/BUILD.gn b/ui/arc/BUILD.gn
index 0849b504f4ad871f2d48f6868c444c3cf27bbf80..31ac7ac4b5ad02350779deaed56f2b5ee0709dd1 100644
--- a/ui/arc/BUILD.gn
+++ b/ui/arc/BUILD.gn
@@ -20,12 +20,19 @@ static_library("arc") {
   ]
 
   deps = [
+    "//ash:ash",
     "//base",
+    "//components/arc:arc_base",
     "//components/arc:arc_bindings",
+    "//components/exo:exo",
     "//components/signin/core/account_id",
     "//skia",
+    "//ui/base",
     "//ui/gfx",
     "//ui/message_center",
+    "//ui/resources",
+    "//ui/strings",
+    "//ui/views",
   ]
 }
 
@@ -42,6 +49,7 @@ test("ui_arc_unittests") {
     "//base/test:test_support",
     "//components/arc:arc_test_support",
     "//mojo/edk/system",
+    "//testing/gtest",
     "//ui/message_center:test_support",
   ]
 }