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

base/message_loop/message_pump_gtk.cc

Index: base/message_loop/message_pump_gtk.cc
diff --git a/base/message_loop/message_pump_gtk.cc b/base/message_loop/message_pump_gtk.cc
index 007434232116b91dd6bdf44dab99b92f97bc5e1f..3279d38fbf2a1a2d7ee1ede9fd435ec570bcc6d2 100644
--- a/base/message_loop/message_pump_gtk.cc
+++ b/base/message_loop/message_pump_gtk.cc
@@ -10,6 +10,8 @@
 #include "base/debug/trace_event.h"
 #include "base/profiler/scoped_profile.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."
+
 namespace base {
 
 namespace {