We want all .h files to be tracked by a GN target. This CL adds a
presubmit check to ensure that newly added targets are tracked by
GN.
In this CL we add a directory called 'presubmit_checks_lib' so we
can avoid to put everything in PRESUBMIT.py (especially if we want
to test some checks). I tried to use 'tools-webrtc' but it is not
easy to include a python module from a directory with '-' in the
name.
BUG=webrtc:7514
Review-Url: https://codereview.webrtc.org/2872493002
Cr-Commit-Position: refs/heads/master@{#18069}
