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/win32window_unittest.cc b/rtc_base/win32window_unittest.cc
index f1d8e84..c659349 100644
--- a/rtc_base/win32window_unittest.cc
+++ b/rtc_base/win32window_unittest.cc
@@ -20,17 +20,17 @@
   const MSG& msg() const { return msg_; }
   bool destroyed() const { return destroyed_; }
 
-  virtual bool OnMessage(UINT uMsg, WPARAM wParam,
-                         LPARAM lParam, LRESULT& result) {
+  bool OnMessage(UINT uMsg,
+                 WPARAM wParam,
+                 LPARAM lParam,
+                 LRESULT& result) override {
     msg_.message = uMsg;
     msg_.wParam = wParam;
     msg_.lParam = lParam;
     result = kDummyResult;
     return true;
   }
-  virtual void OnNcDestroy() {
-    destroyed_ = true;
-  }
+  void OnNcDestroy() override { destroyed_ = true; }
 
  private:
   MSG msg_;