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/thread_unittest.cc b/rtc_base/thread_unittest.cc
index dfaa51a..5f00064 100644
--- a/rtc_base/thread_unittest.cc
+++ b/rtc_base/thread_unittest.cc
@@ -44,7 +44,6 @@
 
 struct TestMessage : public MessageData {
   explicit TestMessage(int v) : value(v) {}
-  virtual ~TestMessage() {}
 
   int value;
 };
@@ -60,9 +59,7 @@
     socket_->SignalReadPacket.connect(this, &SocketClient::OnPacket);
   }
 
-  ~SocketClient() {
-    delete socket_;
-  }
+  ~SocketClient() override { delete socket_; }
 
   SocketAddress address() const { return socket_->GetLocalAddress(); }
 
@@ -90,11 +87,9 @@
       : socket_(socket) {
   }
 
-  virtual ~MessageClient() {
-    delete socket_;
-  }
+  ~MessageClient() override { delete socket_; }
 
-  virtual void OnMessage(Message *pmsg) {
+  void OnMessage(Message* pmsg) override {
     TestMessage* msg = static_cast<TestMessage*>(pmsg->pdata);
     int result = Next(msg->value);
     EXPECT_GE(socket_->Send(&result, sizeof(result)), 0);
@@ -109,7 +104,7 @@
  public:
   CustomThread()
       : Thread(std::unique_ptr<SocketServer>(new rtc::NullSocketServer())) {}
-  virtual ~CustomThread() { Stop(); }
+  ~CustomThread() override { Stop(); }
   bool Start() { return false; }
 
   bool WrapCurrent() {
@@ -129,12 +124,12 @@
       : Thread(std::unique_ptr<SocketServer>(new NullSocketServer())),
         event_(event) {}
 
-  virtual ~SignalWhenDestroyedThread() {
+  ~SignalWhenDestroyedThread() override {
     Stop();
     event_->Set();
   }
 
-  virtual void Run() {
+  void Run() override {
     // Do nothing.
   }