GN: Build with a custom libc++ when using ASAN, TSAN or MSAN.

build/config/sanitizers/BUILD.gn

Index: build/config/sanitizers/BUILD.gn
diff --git a/build/config/sanitizers/BUILD.gn b/build/config/sanitizers/BUILD.gn
index d48ac4e8c07d05503a2317411bca418d930ace55..40d476ea7a5c00926201259e08c41d32944fff5d 100644
--- a/build/config/sanitizers/BUILD.gn
+++ b/build/config/sanitizers/BUILD.gn
@@ -2,15 +2,19 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/sanitizers/sanitizers.gni")
+
 # Contains the dependencies needed for sanitizers to link into executables and
 # shared_libraries. Unconditionally depend upon this target as it is empty if
-# |is_asan|, |is_lsan|, |is_tsan| and |is_msan| are false.
+# |is_asan|, |is_lsan|, |is_tsan|, |is_msan| and |use_custom_libcxx| are false.
 group("deps") {
+  deps = []
   if (is_asan || is_lsan || is_tsan || is_msan) {
     public_configs = [ ":sanitizer_options_link_helper" ]
-    deps = [
-      ":options_sources",
-    ]
+    deps += [ ":options_sources" ]
+  }
+  if (use_custom_libcxx) {
+    deps += [ "//buildtools/third_party/libc++:libcxx_proxy" ]
   }
 }