Add a floating close button for arc custom notification

.gn

 # This file is used by the GN meta build system to find the root of the source
 # tree and to set startup options. For documentation on the values set in this
 # file, run "gn help dotfile" at the command line.
 
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
 # The secondary source root is a parallel directory tree where
 # GN build files are placed when they can not be placed directly
 # in the source tree, e.g. for third party source trees.
@@ skipping 108 matching lines @@
   "//third_party/kasko/*",
   "//third_party/libaddressinput/*",
   "//third_party/libphonenumber/*",
   "//third_party/WebKit/Source/*",
   "//tools/*",
 
   #"//ui/*",  # Work left on Chromeos w/ use_ozone. Some parts of UI that work:
   "//ui/accessibility/*",
   "//ui/android/*",
   "//ui/app_list/*",
+  "//ui/arc/*",
   "//ui/aura/*",
   "//ui/aura_extra/*",
   "//ui/base/*",
   "//ui/chromeos/*",
   "//ui/compositor/*",
   "//ui/content_accelerators/*",
   "//ui/display/*",
   "//ui/events/*",
   "//ui/file_manager/*",
   "//ui/gfx/*",
@@ skipping 189 matching lines @@
   "//tools/gn/input_conversion.cc",
   "//tools/gn/misc/emacs/gn.el",
   "//tools/gn/misc/vim/syntax/gn.vim",
   "//tools/gn/setup.cc",
   "//ui/accessibility/BUILD.gn",
   "//ui/views/BUILD.gn",
   "//ui/views/mus/BUILD.gn",
   "//v8/test/cctest/BUILD.gn",
   "//v8/test/unittests/BUILD.gn",
 ]