Add ubsan bug reference to GN build.

ubsan is not supported.

TBR=dpranke

Review URL: https://codereview.chromium.org/1330523002

Cr-Original-Commit-Position: refs/heads/master@{#347003}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8c0528e49adf32b5255c433bb135a2139e7c9ae7

config/compiler/BUILD.gn

@@ -84,7 +84,7 @@ config("default_include_dirs") {
   ]
 }
 
-# TODO(GYP): is_ubsan, is_ubsan_vptr
+# TODO(GYP) bug 527515: is_ubsan, is_ubsan_vptr
 if (!is_win) {
   using_sanitizer = is_asan || is_lsan || is_tsan || is_msan
 }