Make GN use_custom_libcxx work properly with sanitizers.

build/config/sanitizers/sanitizers.gni

 # Copyright 2015 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.
 
 declare_args() {
   # Compile for Address Sanitizer to find memory bugs.
   is_asan = false
 
   # Compile for Leak Sanitizer to find leaks.
   is_lsan = false
 
   # Compile for Memory Sanitizer to find uninitialized reads.
   is_msan = false
 
   # 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 || 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
   # chain of stores leading from allocation site to use site.
   msan_track_origins = 2
 
   # Use dynamic libraries instrumented by one of the sanitizers instead of the
   # standard system libraries. Set this flag to download prebuilt binaries from
   # GCS.
   use_prebuilt_instrumented_libraries = false
 
   # Enable building with SyzyAsan which can find certain types of memory
   # errors. Only works on Windows. See
   # https://code.google.com/p/sawbuck/wiki/SyzyASanHowTo
   is_syzyasan = false
 
   # Compile with Control Flow Integrity to protect virtual calls and casts.
   # See http://clang.llvm.org/docs/ControlFlowIntegrity.html
   is_cfi = false
 
   # By default, Control Flow Integrity will crash the program if it detects a
   # violation. Set this to true to print detailed diagnostics instead.
   use_cfi_diag = false
 }
 
+# Args that are in turn dependent on other args must be in a separate
+# declare_args block. User overrides are only applied at the end of a
+# declare_args block.
+declare_args() {
+  # 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 || is_ubsan
+}
+
 # TODO(GYP) bug 527515: is_ubsan, is_ubsan_vptr
 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'")
 
 # MSan only links Chrome properly in release builds (brettw -- 9/1/2015). The
 # same is possibly true for the other non-ASan sanitizers. But regardless of
 # whether it links, one would normally never run a sanitizer in debug mode.
 # Running in debug mode probably indicates you forgot to set the "is_debug =
 # false" flag in the build args. ASan seems to run fine in debug mode.
 #
 # If you find a use-case where you want to compile a sanitizer in debug mode
 # and have verified it works, ask brettw and we can consider removing it from
 # this condition. We may also be able to find another way to enable your case
 # without having people accidentally get broken builds by compiling an
 # unsupported or unadvisable configurations.
 #
 # For one-off testing, just comment this assertion out.
 assert(!is_debug || !(is_msan || is_lsan || is_tsan || is_ubsan),
        "Sanitizers should generally be used in release (set is_debug=false).")