Reland "Enable the clang style plugin in rtc_base/"

This is a reland of Id63f0deb7b335690157ab157c35177b7836688da.

Original change's description:
> Enable the clang style plugin in rtc_base/
> 
> Enabled the plugin and cleaned up all issues it found.
> 
> Bug: webrtc:163
> Change-Id: Id63f0deb7b335690157ab157c35177b7836688da
> Reviewed-on: https://webrtc-review.googlesource.com/14660
> Commit-Queue: Steve Anton <steveanton@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20401}

Bug: webrtc:163
Change-Id: I861a5fe741215115b0e7a2be9c0786836ff5376e
Reviewed-on: https://webrtc-review.googlesource.com/15040
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20416}
diff --git a/rtc_base/nullsocketserver_unittest.cc b/rtc_base/nullsocketserver_unittest.cc
index 29f4980..e3d9952 100644
--- a/rtc_base/nullsocketserver_unittest.cc
+++ b/rtc_base/nullsocketserver_unittest.cc
@@ -18,12 +18,9 @@
 class NullSocketServerTest
     : public testing::Test,
       public MessageHandler {
- public:
-  NullSocketServerTest() {}
  protected:
-  virtual void OnMessage(Message* message) {
-    ss_.WakeUp();
-  }
+  void OnMessage(Message* message) override { ss_.WakeUp(); }
+
   NullSocketServer ss_;
 };