Style cleanup in preparation for auto-linting base/.

base/waitable_event_watcher_unittest.cc

Index: base/waitable_event_watcher_unittest.cc
diff --git a/base/waitable_event_watcher_unittest.cc b/base/waitable_event_watcher_unittest.cc
index c1d930a80c3ceb87ae8f2b2510c8c2b8471a302d..86acee066bba6fdfe53278ab4e17bf7513b9e63b 100644
--- a/base/waitable_event_watcher_unittest.cc
+++ b/base/waitable_event_watcher_unittest.cc
@@ -22,7 +22,7 @@ class QuitDelegate : public WaitableEventWatcher::Delegate {
 
 class DecrementCountDelegate : public WaitableEventWatcher::Delegate {
  public:
-  DecrementCountDelegate(int* counter) : counter_(counter) {
+  explicit DecrementCountDelegate(int* counter) : counter_(counter) {
   }
   virtual void OnWaitableEventSignaled(WaitableEvent* object) {
     --(*counter_);