Enable cpplint for webrtc/examples and fix all uncovered cpplint errors.

BUG=webrtc:5273
TESTED=Fixed issues reported by:
find webrtc/examples/ -type f -name *.cc -o -name *.h | grep -v objc | xargs cpplint.py
followed by 'git cl presubmit'.

NOTRY=True

Review URL: https://codereview.webrtc.org/1504283004

Cr-Commit-Position: refs/heads/master@{#10960}

webrtc/examples/stunserver/stunserver_main.cc

@@ -17,7 +17,7 @@
 #include "webrtc/p2p/base/stunserver.h"
 #include "webrtc/base/thread.h"
 
-using namespace cricket;
+using cricket::StunServer;
 
 int main(int argc, char* argv[]) {
   if (argc != 2) {