Add presubmit check to prevent package boundary violations.

tools-webrtc/testdata/BUILD.gn

Index: tools-webrtc/testdata/BUILD.gn
diff --git a/tools-webrtc/testdata/BUILD.gn b/tools-webrtc/testdata/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..3556e509e7fc66a342c48830fd69b1f2d26c28a5
--- /dev/null
+++ b/tools-webrtc/testdata/BUILD.gn
@@ -0,0 +1,22 @@
+# Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
+#
+# Use of this source code is governed by a BSD-style license
+# that can be found in the LICENSE file in the root of the source
+# tree. An additional intellectual property rights grant can be found
+# in the file PATENTS.  All contributing project authors may
+# be found in the AUTHORS file in the root of the source tree.
+
+group("testdata") {
+  deps = [
+    ":one",
+  ]
+}
+
+static_library("dummy_target") {
+  sources = [
+    "dummy.cc",
+  ]
+  deps = [
+    "subdir",
+  ]
+}