Style cleanup in preparation for auto-linting base/.

base/at_exit.h

Index: base/at_exit.h
diff --git a/base/at_exit.h b/base/at_exit.h
index 48ba3d9c71de300c7d509f1f132614c808139b98..082525ea2b279bb1b665b4fb64b3f3b7e610b4b8 100644
--- a/base/at_exit.h
+++ b/base/at_exit.h
@@ -32,7 +32,7 @@ class AtExitManager {
   // even if one already exists.  This should only be used for testing!
   // AtExitManagers are kept on a global stack, and it will be removed during
   // destruction.  This allows you to shadow another AtExitManager.
-  AtExitManager(bool shadow);
+  explicit AtExitManager(bool shadow);
 
  public:
   typedef void (*AtExitCallbackType)(void*);