Attempt to fix Mac ASAN build in GN.

build/config/sanitizers/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/clang/clang.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ skipping 49 matching lines @@
     if (use_prebuilt_instrumented_libraries ||
         use_locally_built_instrumented_libraries) {
       deps += [ "//third_party/instrumented_libraries:deps" ]
     }
     if (use_custom_libcxx) {
       public_deps = [
         "//buildtools/third_party/libc++:libcxx_proxy",
       ]
       data += [ "$root_out_dir/libc++.so" ]
     }
+
     if (is_mac) {
       data_deps = [
         ":copy_asan_runtime",
       ]
     }
   }
 }
 
 if (is_mac && using_sanitizer) {
-  copy("copy_asan_runtime") {
+  action("copy_asan_runtime") {
+    script = "//build/config/sanitizers/copy_asan_runtime_mac.py"
+    _basename = "libclang_rt.asan_osx_dynamic.dylib"
     sources = [
-      "$clang_base_path/lib/clang/$clang_version/lib/darwin/libclang_rt.asan_osx_dynamic.dylib",
+      "$clang_base_path/lib/clang/$clang_version/lib/darwin/$_basename",
     ]
     outputs = [
-      "$root_out_dir/{{source_file_part}}",
+      "$root_build_dir/$_basename",
     ]
+    args = rebase_path(sources, root_build_dir) +
+           rebase_path(outputs, root_build_dir)
   }
 }
 
 config("sanitizer_options_link_helper") {
   if (is_mac) {
     ldflags = [ "-Wl,-U,_sanitizer_options_link_helper" ]
   } else if (!is_win) {
     ldflags = [ "-Wl,-u_sanitizer_options_link_helper" ]
   }
 }
@@ skipping 336 matching lines @@
 # This allows to selectively disable ubsan_vptr, when needed. In particular,
 # if some third_party code is required to be compiled without rtti, which
 # is a requirement for ubsan_vptr.
 config("default_sanitizer_flags_but_ubsan_vptr") {
   configs = all_sanitizer_configs - [ ":ubsan_vptr_flags" ]
 }
 
 config("default_sanitizer_flags_but_coverage") {
   configs = all_sanitizer_configs - [ ":coverage_flags" ]
 }