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..c5082dfa237b586d6c191b8d997382d1407bf252 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/libgtk2ui:libgtk3ui",

[Dirk Pranke, 2016/10/25 23:56:10]

You're not really creating a target called libgtk3ui in a directory called
libgtk2ui, are you? :)

Can we rename the directory?

[Tom (Use chromium acct), 2016/10/26 18:20:01]

Ah, well I guess it had to be done eventually, yay sed :)

Note that there are still files within libgtkui with gtk2 in their name.  Maybe
this would be better for a separate refactoring CL.

     "//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/libgtk2ui:libgtk3ui" ]
   public_configs = [ ":gtkprint3_internal_config" ]
 }