Attempt to fix Mac ASAN build in GN.

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 dff6eb3c96612b2e294ac63f0abca1fe961e6ebc..5964e291c657b346356e05cac8cb11d5d2e56f81 100644
--- a/build/config/sanitizers/BUILD.gn
+++ b/build/config/sanitizers/BUILD.gn
@@ -67,6 +67,7 @@ group("deps_no_options") {
       ]
       data += [ "$root_out_dir/libc++.so" ]
     }
+
     if (is_mac) {
       data_deps = [
         ":copy_asan_runtime",
@@ -76,13 +77,17 @@ group("deps_no_options") {
 }
 
 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)
   }
 }