Move GN's glib_cxx_debug flag onto the debug configuration.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index ce0aae7afb72d0def54b1ec6b0713cd0c197d5bc..10617cd18f7a0f700c9adda97721f271bdedffed 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -128,11 +128,6 @@ config("compiler") {
     }
 
     if (cpu_arch == "x64") {
-      # Enable libstdc++ debugging facilities to help catch problems early, see
-      # http://crbug.com/65151 .
-      # TODO(phajdan.jr): Should we enable this for all of POSIX?
-      defines += [ "_GLIBCXX_DEBUG=1" ]
-
       # Use gold for linking on 64-bit Linux only (on 32-bit it runs out of
       # address space, and it doesn't support cross-compiling).
       gold_path = rebase_path("//third_party/gold", ".", root_build_dir)