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..79c7c3578b12cc9f905e1e498ef6cc850a97425f 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/libgtkui:libgtk3ui" ]
+    } else {
+      deps += [ "//chrome/browser/ui/libgtkui:libgtk2ui" ]
+    }
   }
   if (is_posix && !is_mac) {
     sources += [
@@ -2364,7 +2367,6 @@ split_static_library("browser") {
       "themes/custom_theme_supplier.cc",
       "themes/custom_theme_supplier.h",
       "themes/theme_properties.cc",
-      "themes/theme_properties.h",
       "themes/theme_service.cc",
       "themes/theme_service.h",
       "themes/theme_service_factory.cc",
@@ -2373,6 +2375,7 @@ split_static_library("browser") {
       "themes/theme_syncable_service.cc",
       "themes/theme_syncable_service.h",
     ]
+    deps += [ ":theme_properties" ]
   }
 
   if (enable_basic_printing || enable_print_preview) {
@@ -3963,6 +3966,12 @@ if (is_win) {
   }
 }
 
+source_set("theme_properties") {
+  sources = [
+    "themes/theme_properties.h",
+  ]
+}
+
 proto_library("resource_prefetch_predictor_proto") {
   sources = [
     "predictors/resource_prefetch_predictor.proto",