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

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index fc728a044ed07c3b94b2af1c3bd0c6717cf7a8af..2cfde5b9210184f40a4138f1a6f38ee1ba146aac 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -170,8 +170,14 @@ config("compiler") {
         "-fsanitize-memory-track-origins=$msan_track_origins",
         "-fsanitize-blacklist=$msan_blacklist_path",
       ]
+    }
 
-      # TODO(GYP): Support custom libc++.
+    if (use_custom_libcxx) {
+      cflags_cc += [ "-nostdinc++" ]
+      include_dirs = [
+        "//buildtools/third_party/libc++/trunk/include",
+        "//buildtools/third_party/libc++abi/trunk/include",
+      ]
     }
   }