Clean up GTK3 build flags.

build/config/linux/gtk/BUILD.gn

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/linux/gtk/gtk.gni")
+import("//build/config/linux/pkg_config.gni")
+
+assert(is_linux, "This file should only be referenced on Linux")
+
+# The target in this file will automatically reference GTK2 or GTK3 depending
+# on the state of the build flag. Some builds reference both 2 and 3, and some
+# builds reference neither, so both need to be available but in different
+# directories so pkg-config is only run when necessary.
+
+# Basically no parts of Chrome should depend on GTK. To prevent accidents, the
+# parts that explicitly need GTK are whitelisted on this target.
+group("gtk") {
+  visibility = [
+    "//chrome/test:interactive_ui_tests",
+    "//gpu/gles2_conform_support:gles2_conform_test_windowless",
+    "//remoting/host",
+    "//remoting/host/it2me:common",
+    "//remoting/host/it2me:remote_assistance_host",
+    "//remoting/host:remoting_me2me_host_static",
+    "//remoting/test:it2me_standalone_host_main",
+  ]
+
+  if (use_gtk3) {
+    public_deps = [
+      "//build/config/linux/gtk3",
+    ]
+  } else {
+    public_deps = [
+      "//build/config/linux/gtk2",
+    ]
+  }
+}