Add copyright headers to remaining gyp files.

Prevents some PRESUBMIT errors.

TBR=mtklein@google.com

Review URL: https://codereview.chromium.org/1035523003
diff --git a/gyp/bench.gypi b/gyp/bench.gypi
index 3b77fc5..2efcec2 100644
--- a/gyp/bench.gypi
+++ b/gyp/bench.gypi
@@ -1,3 +1,7 @@
+# Copyright 2015 Google Inc.
+#
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
 {
   'include_dirs': [
     '../src/core',