Add GN configs for -fsanitizer=null.

build/toolchain/gcc_toolchain.gni

 # Copyright (c) 2013 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/nacl/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ skipping 464 matching lines @@
       }
 
       # Disable sanitizers for non-default toolchains.
       is_asan = false
       is_cfi = false
       is_lsan = false
       is_msan = false
       is_syzyasan = false
       is_tsan = false
       is_ubsan = false
+      is_ubsan_null = false
       is_ubsan_vptr = false
     }
 
     forward_variables_from(invoker, [ "deps" ])
   }
 }
 
 # This is a shorthand for gcc_toolchain instances based on the
 # Chromium-built version of Clang.  Only the toolchain_cpu and
 # toolchain_os variables need to be specified by the invoker, and
@@ skipping 30 matching lines @@
                              "toolchain_os",
                              "use_gold",
                              "strip",
                            ])
 
     if (defined(invoker.use_debug_fission)) {
       use_debug_fission = invoker.use_debug_fission
     }
   }
 }