Add presubmit check to prevent package boundary violations.

tools-webrtc/testdata/multiple_errors_multiple_targets/BUILD.gn

+# 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.
+
+target("dummy_target_ok") {
+  sources = [
+    "dummy_source.cc",
+    "dummy_source.h",
+  ]
+}
+
+target("error_1") {
+  sources = [
+    "subpackage1/dummy_subpackage1.cc",
+    "subpackage1/dummy_subpackage1.h",
+  ]
+}
+
+target("error_2") {
+  sources = [
+    "subpackage1/dummy_subpackage2.cc",
+    "subpackage1/dummy_subpackage2.h",
+  ]
+}