Add presubmit check to prevent package boundary violations.

tools-webrtc/testdata/expected_stderr

Index: tools-webrtc/testdata/expected_stderr
diff --git a/tools-webrtc/testdata/expected_stderr b/tools-webrtc/testdata/expected_stderr
new file mode 100644
index 0000000000000000000000000000000000000000..3edb940a1b3b46dcac0ee8405e02ad4cd1fec387
--- /dev/null
+++ b/tools-webrtc/testdata/expected_stderr
@@ -0,0 +1,12 @@
+testdata/subdir/BUILD.gn:21 in target 'package_boundary_violation':

[kjellander_webrtc, 2017/01/24 09:33:19]

I think it's better to write more fine-grained test cases so we don't need to
resort to just comparing stdout/stderr. When a test fails it's likely going to
be a harder-to-read failure message with this approach.

+  Source file 'subdir2/dummy4.cc'
+  crosses boundary of package 'subdir2'.
+
+testdata/subdir/BUILD.gn:22 in target 'package_boundary_violation':
+  Source file 'subdir2/dummy4.h'
+  crosses boundary of package 'subdir2'.
+
+testdata/subdir/BUILD.gn:34 in target 'package_boundary_violation_unittests':
+  Source file 'subdir2/dummy3_unittest.cc'
+  crosses boundary of package 'subdir2'.
+