Standardize usage of virtual/override/final in base/

base/synchronization/waitable_event_unittest.cc

Index: base/synchronization/waitable_event_unittest.cc
diff --git a/base/synchronization/waitable_event_unittest.cc b/base/synchronization/waitable_event_unittest.cc
index a8913fd30a41a713cec449bbe0d71dd9208ba0d6..abba9356bb17a2e967863a2e06e0a6a4dfaed092 100644
--- a/base/synchronization/waitable_event_unittest.cc
+++ b/base/synchronization/waitable_event_unittest.cc
@@ -78,7 +78,7 @@ class WaitableEventSignaler : public PlatformThread::Delegate {
         ev_(ev) {
   }
 
-  virtual void ThreadMain() override {
+  void ThreadMain() override {
     PlatformThread::Sleep(TimeDelta::FromSecondsD(seconds_));
     ev_->Signal();
   }