GTK: Adds #error statements to GTK code to root out unknown bots.

ui/surface/transport_dib_gtk.cc

Index: ui/surface/transport_dib_gtk.cc
diff --git a/ui/surface/transport_dib_gtk.cc b/ui/surface/transport_dib_gtk.cc
index 61ec624c06582532dfd14299a28f0323c38845e9..0d075a18ac67c7948a2e7bb3f6f57bd6fcadcf8d 100644
--- a/ui/surface/transport_dib_gtk.cc
+++ b/ui/surface/transport_dib_gtk.cc
@@ -17,6 +17,8 @@
 #include "ui/base/x/x11_util.h"
 #include "ui/gfx/size.h"
 
+#error "The GTK+ port will be deleted later this week. If you are seeing this, you are trying to compile it. Please check your gyp flags for 'use_aura=0' and remove them."
+
 // The shmat system call uses this as it's invalid return address
 static void *const kInvalidAddress = (void*) -1;