This script is similar to header_usage.sh, but it counts usage (number
of files) of each class defined in some header file.
E.g., the following classes appear unused,
AsyncHttpsProxyServerSocket
CompositeMediaEngineWithFakeVoiceEngine
DesktopId
RtpDataCallback
ScopedMessageData
ScreencastEventCatcher
NOTRY=true
BUG=None
Review-Url: https://codereview.webrtc.org/2649103002
Cr-Commit-Position: refs/heads/master@{#16204}
