Switch Mac to in-tree libc++ builds

Binary size regression of up to 512KiB is expected.

buildtools roll includes a single CL:
2f02e1f363

Note on chromium-dev:
https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/QeXzX4MVrlo

BUG=840822,923084
R=thakis
TBR=kbr

Change-Id: Iacaf90dc480c6d8b9939c99c398a8960a2e9d4ed
Reviewed-on: https://chromium-review.googlesource.com/c/1396842
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#624193}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d9d71d7e9f8ebd301d9a6449adfef44e63a65487

config/c++/c++.gni

@@ -5,13 +5,13 @@
 import("//build/config/sanitizers/sanitizers.gni")
 
 declare_args() {
-  # Use libc++ (buildtools/third_party/libc++ and
-  # buildtools/third_party/libc++abi) instead of stdlibc++ as standard
-  # library.
+  # Use in-tree libc++ (buildtools/third_party/libc++ and
+  # buildtools/third_party/libc++abi) instead of the system C++ library for C++
+  # standard library support.
   # Don't check in changes that set this to false for more platforms; doing so
   # is not supported.
   use_custom_libcxx =
-      is_msan || is_fuchsia || is_android ||
+      is_fuchsia || is_android || is_mac ||
       (is_linux &&
        (!is_chromeos || default_toolchain != "//build/toolchain/cros:target"))
 

config/mac/BUILD.gn

@@ -2,9 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/config/sysroot.gni")
+import("//build/config/c++/c++.gni")
 import("//build/config/mac/mac_sdk.gni")
 import("//build/config/mac/symbols.gni")
+import("//build/config/sysroot.gni")
 
 # This is included by reference in the //build/config/compiler config that
 # is applied to all targets. It is here to separate out the logic.
@@ -51,6 +52,10 @@ config("compiler") {
   if (save_unstripped_output) {
     ldflags += [ "-Wcrl,unstripped," + rebase_path(root_out_dir) ]
   }
+
+  if (export_libcxxabi_from_executables) {
+    ldflags += [ "-Wl,-undefined,dynamic_lookup" ]
+  }
 }
 
 # This is included by reference in the //build/config/compiler:runtime_library