Add presubmit check to prevent package boundary violations.

tools-webrtc/testdata/subdir/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.
+
+static_library("subdir") {
+  sources = [
+    "dummy2.cc",
+  ]
+  deps = [
+    ":package_boundary_violation",
+    "subdir2",
+  ]
+}
+
+static_library("package_boundary_violation") {
+  sources = [
+    "subdir2/dummy4.cc",
+    "subdir2/dummy4.h",
+  ]
+  deps = [
+    "subdir2",
+  ]
+}
+
+# Testing that this work with indentation.
+if (is_win) {
+  source_set("package_boundary_violation_unittests") {
+    testonly = true
+    sources = [
+      "subdir2/dummy3_unittest.cc",
+    ]
+    deps = [
+      "subdir2:dummy_test_support",
+    ]
+  }
+}