This target keeps track of .h the files under webrtc/ that are not part
of any target.
If a .h file is not part of a target the 'gn check' utility is not
able to spot if a target is missing a dependency because even if
it parses '#include' directives it is not able to find a target that
contains these headers.
BUG=webrtc:7512
NOTRY=True
Review-Url: https://codereview.webrtc.org/2841873002
Cr-Commit-Position: refs/heads/master@{#17874}
81c79f51aa