Add Undefined Behaviour sanitizer support to GN.

build/config/sanitizers/sanitizers.gni

Index: build/config/sanitizers/sanitizers.gni
diff --git a/build/config/sanitizers/sanitizers.gni b/build/config/sanitizers/sanitizers.gni
index 1aa212deab712df718a92d6bf0e00315348d2ab3..84348da425ae90a53e5f74edbf7709f51fd1cd50 100644
--- a/build/config/sanitizers/sanitizers.gni
+++ b/build/config/sanitizers/sanitizers.gni
@@ -15,10 +15,14 @@ declare_args() {
   # Compile for Thread Sanitizer to find threading bugs.
   is_tsan = false
 
+  # Compile for Undefined Behaviour Sanitizer to find various types of
+  # undefined behaviour.
+  is_ubsan = false
+
   # Use libc++ (buildtools/third_party/libc++ and
   # buildtools/third_party/libc++abi) instead of stdlibc++ as standard library.
   # This is intended to be used for instrumented builds.
-  use_custom_libcxx = (is_asan && is_linux) || is_tsan || is_msan
+  use_custom_libcxx = (is_asan && is_linux) || is_tsan || is_msan || is_ubsan
 
   # Track where uninitialized memory originates from. From fastest to slowest:
   # 0 - no tracking, 1 - track only the initial allocation site, 2 - track the
@@ -45,7 +49,7 @@ declare_args() {
 }
 
 # TODO(GYP) bug 527515: is_ubsan, is_ubsan_vptr
-using_sanitizer = is_asan || is_lsan || is_tsan || is_msan
+using_sanitizer = is_asan || is_lsan || is_tsan || is_msan || is_ubsan
 
 assert(!using_sanitizer || is_clang,
        "Sanitizers (is_*san) require setting is_clang = true in 'gn args'")
@@ -63,5 +67,5 @@ assert(!using_sanitizer || is_clang,
 # unsupported or unadvisable configurations.
 #
 # For one-off testing, just comment this assertion out.
-assert(!is_debug || !(is_msan || is_lsan || is_tsan),
+assert(!is_debug || !(is_msan || is_lsan || is_tsan || is_ubsan),
        "Sanitizers should generally be used in release (set is_debug=false).")