Allow GLES2 conform to use GTK2

cdf496ad123d8b43452d65a81f76cbd26d348ebf breaks the
generate_build_files of the GPU Linux builder that has a special flag
enabling compilation of the gles2_conform tests.

TBR=brettw@chromium.org

Review-Url: https://codereview.chromium.org/2516313003
Cr-Original-Commit-Position: refs/heads/master@{#433698}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 420f6d042815895b95a5200edee2214e4eb9029f

config/linux/gtk2/BUILD.gn

@@ -26,6 +26,7 @@ pkg_config("gtk2_internal_config") {
 # parts that explicitly need GTK2 are whitelisted on this target.
 group("gtk2") {
   visibility = [
+    "//gpu/gles2_conform_support:gles2_conform_test_windowless",
     "//build/config/linux/gtk",
     "//chrome/browser/ui/libgtkui:*",
   ]