Gtk3 ui: Add libgtk3ui as a separate build component

build/config/linux/gtk2/BUILD.gn

Index: build/config/linux/gtk2/BUILD.gn
diff --git a/build/config/linux/gtk2/BUILD.gn b/build/config/linux/gtk2/BUILD.gn
index 3375e35ed5e53b7894d42025d5e7d059155365d0..861d4fb746cd6f4965a74ab92a68e689204346a4 100644
--- a/build/config/linux/gtk2/BUILD.gn
+++ b/build/config/linux/gtk2/BUILD.gn
@@ -25,7 +25,7 @@ pkg_config("gtk2_internal_config") {
 # parts that explicitly need GTK are whitelisted on this target.
 group("gtk2") {
   visibility = [
-    "//chrome/browser/ui/libgtk2ui",
+    "//chrome/browser/ui/libgtkui:libgtk2ui",
     "//gpu/gles2_conform_support:gles2_conform_test_windowless",
     "//remoting/host",
     "//remoting/host/it2me:common",
@@ -42,6 +42,6 @@ pkg_config("gtkprint2_internal_config") {
 }
 
 group("gtkprint2") {
-  visibility = [ "//chrome/browser/ui/libgtk2ui" ]
+  visibility = [ "//chrome/browser/ui/libgtkui:libgtk2ui" ]
   public_configs = [ ":gtkprint2_internal_config" ]
 }