Add a target in GN that generates a last_change.h file for the SVN revision.

tools/gn/secondary/build/util/BUILD.gn

Index: tools/gn/secondary/build/util/BUILD.gn
diff --git a/tools/gn/secondary/build/util/BUILD.gn b/tools/gn/secondary/build/util/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..d43cc4907aa20a0543501fa3c743c0eb15d35f7f
--- /dev/null
+++ b/tools/gn/secondary/build/util/BUILD.gn
@@ -0,0 +1,35 @@
+# 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.
+
+# This target generates a "last_change.h" header file in the generated files
+# directory that contains a define of the last revision of the source tree
+# of the form:
+#  #define LAST_CHANGE "123456"
+#
+# The version is a string rather than an integer for extra flexibility (for
+# example, we may require git hashes in the future).
+#
+# All you nede to do is depend on this target, and then from your source code:
+#  #include "build/util/last_change.h"
+custom("last_change") {
+  script = "//build/util/lastchange.py"
+
+  # This script must be run before targets depending on us.
+  hard_dep = true
+
+  # Rerun the script any time this file changes.
+  source_prereqs = [ "//build/util/LASTCHANGE" ]
+
+  output_header = "$target_gen_dir/last_change.h"
+  outputs = [ output_header ]
+
+  build_relative_src = rebase_path("//", ".", root_build_dir)
+  build_relative_outputs = rebase_path(output_header, ".", root_build_dir)
+
+  args = [
+    "--source-dir=$build_relative_src",
+    "--header=$build_relative_outputs",
+    "--version-macro=LAST_CHANGE",
+  ]
+}