Gtk3 ui: Add libgtk3ui as a separate build component

chrome/browser/BUILD.gn

Index: chrome/browser/BUILD.gn
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index eeb861e22c622b4defaac1402068d876e2686f2b..fff52aace9ee744024bdd9395d8aa51461833cc1 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -2237,8 +2237,11 @@ split_static_library("browser") {
     ]
   }
   if (use_aura && !use_ozone && is_desktop_linux) {
-    deps += [ "//chrome/browser/ui/libgtk2ui" ]
-    allow_circular_includes_from += [ "//chrome/browser/ui/libgtk2ui" ]
+    if (use_gtk3) {
+      deps += [ "//chrome/browser/ui/libgtk2ui:libgtk3ui" ]
+    } else {
+      deps += [ "//chrome/browser/ui/libgtk2ui:libgtk2ui" ]
+    }
   }
   if (is_posix && !is_mac) {
     sources += [