GN: Factor "stamp" and "copy" details out of toolchains

build/toolchain/toolchain.gni

 # Copyright 2015 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.
 
 # Toolchain-related configuration that may be needed outside the context of the
 # toolchain() rules themselves.
 
 # Subdirectory within root_out_dir for shared library files.
 # TODO(agrieve): GYP sets this to "lib" for Linux & Android, but this won't work
 #     in GN until support for loadable_module() is added.
@@ skipping 20 matching lines @@
 } else {
   assert(false, "Platform not supported")
 }
 
 # Prefix for shared library files.
 if (is_posix) {
   shlib_prefix = "lib"
 } else {
   shlib_prefix = ""
 }
+
+# While other "tool"s in a toolchain are specific to the target of that
+# toolchain, the "stamp" and "copy" tools are really generic to the host;
+# but each toolchain must define them separately.  GN doesn't allow a
+# template instantiation inside a toolchain definition, so some boilerplate
+# has to be repeated in each toolchain to define these two tools.  These
+# four variables reduce the duplication in that boilerplate.
+stamp_description = "STAMP {{output}}"
+copy_description = "COPY {{source}} {{output}}"
+if (host_os == "win") {
+  stamp_command = "$python_path gyp-win-tool stamp {{output}}"
+  copy_command =
+      "$python_path gyp-win-tool recursive-mirror {{source}} {{output}}"
+} else {
+  stamp_command = "touch {{output}}"
+  copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
+}