Add Undefined Behaviour sanitizer support to GN.

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/ccache.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ skipping 335 matching lines @@
                                "is_nacl_glibc",
                              ])
 
       if (defined(invoker.clear_sanitizers) && invoker.clear_sanitizers) {
         is_asan = false
         is_cfi = false
         is_lsan = false
         is_msan = false
         is_syzyasan = false
         is_tsan = false
+        is_ubsan = false
       }
     }
 
     forward_variables_from(invoker, [ "deps" ])
   }
 }