Gtk3 ui: Add libgtk3ui as a separate build component

build/config/linux/gtk3/BUILD.gn

Index: build/config/linux/gtk3/BUILD.gn
diff --git a/build/config/linux/gtk3/BUILD.gn b/build/config/linux/gtk3/BUILD.gn
index db218edc4d28b2ebe29dbd429c49d001a76ed00d..69974c1e6c5da266e4d1ac1895916cf0e5b2b198 100644
--- a/build/config/linux/gtk3/BUILD.gn
+++ b/build/config/linux/gtk3/BUILD.gn
@@ -25,7 +25,7 @@ pkg_config("gtk3_internal_config") {
 # parts that explicitly need GTK are whitelisted on this target.
 group("gtk3") {
   visibility = [
-    "//chrome/browser/ui/libgtk2ui",
+    "//chrome/browser/ui/libgtkui:libgtk3ui",
     "//gpu/gles2_conform_support:gles2_conform_test_windowless",
     "//remoting/host",
     "//remoting/host/it2me:common",
@@ -42,6 +42,6 @@ pkg_config("gtkprint3_internal_config") {
 }
 
 group("gtkprint3") {
-  visibility = [ "//chrome/browser/ui/libgtk2ui" ]
+  visibility = [ "//chrome/browser/ui/libgtkui:libgtk3ui" ]
   public_configs = [ ":gtkprint3_internal_config" ]
 }