Update GN bootstrap build

tools/gn/bootstrap/base/generated_build_date.h

Index: tools/gn/bootstrap/base/generated_build_date.h
diff --git a/tools/gn/bootstrap/base/generated_build_date.h b/tools/gn/bootstrap/base/generated_build_date.h
new file mode 100644
index 0000000000000000000000000000000000000000..369e68fbdbba901b03dfa2b864045e5040f722bd
--- /dev/null
+++ b/tools/gn/bootstrap/base/generated_build_date.h
@@ -0,0 +1,12 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.

[tfarina, 2016/02/16 14:47:05]

Btw, how come build_date.cc is seeing this file in tools/gn/bootstrap? Is
tools/gn/bootstrap being included in -I path? If so, could you show me where?
Otherwise, I don't see how is is possible be working. Perhaps paste the command
line that bootstrap is using to compile it, just for us to see what it is doing.

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// When building Chromium with GN, this file is generated by
+// the //base:build_date action and it contains an approximate build date.
+// For GN bootstrapping we still need this file to be available
+// because it is included from base/build_time.cc, but the actual
+// BUILD_DATE define is given as a compiler option set by bootstrap.py.
+#ifndef BUILD_DATE
+#error BUILD_DATE must be defined by bootstrap.py
+#endif // BUILD_DATE