Add the concept of a source set to GN.

tools/gn/secondary/build/toolchain/nacl/BUILD.gn

 # Copyright (c) 2013 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("x86_newlib") {
   toolprefix = "gen/sdk/toolchain/linux_x86_newlib/bin/x86_64-nacl-"
   cc = toolprefix + "gcc"
   cxx = toolprefix + "g++"
   ld = toolprefix + "g++"
@@ skipping 20 matching lines @@
     description = "SOLINK(NaCl x86 Newlib) \$lib"
     #pool = "link_pool"
     restat = "1"
   }
   tool("link") {
     command = "$ld \$ldflags -o \$out -Wl,--start-group \$in \$solibs -Wl,--end-group \$libs"
     description = "LINK(NaCl x86 Newlib) \$out"
     #pool = "link_pool"
   }
 
+  if (is_win) {
+    tool("stamp") {
+      command = "$python_path gyp-win-tool stamp \$out"
+      description = "STAMP \$out"
+    }
+  } else {
+    tool("stamp") {
+      command = "touch \$out"
+      description = "STAMP \$out"
+    }
+  }
+
   toolchain_args() {
     is_nacl = true
 
     # Component build not supported in NaCl, since it does not support shared
     # libraries.
     is_component_build = false
 
     # Override the default OS detection.
     is_android = false
     is_mac = false
     is_posix = true
     is_linux = false
     is_win = false
   }
 }