Add missing (and remove superfluous) 'explicit' from constructors.

base/threading/thread_collision_warner.h

Index: base/threading/thread_collision_warner.h
diff --git a/base/threading/thread_collision_warner.h b/base/threading/thread_collision_warner.h
index 2181cbd8dab0959d98286ede785f376ab778d34b..d509a58e8dee4bcef2adc5aba0bcf768e0e2a3fd 100644
--- a/base/threading/thread_collision_warner.h
+++ b/base/threading/thread_collision_warner.h
@@ -144,7 +144,7 @@ struct BASE_EXPORT DCheckAsserter : public AsserterBase {
 class BASE_EXPORT ThreadCollisionWarner {
  public:
   // The parameter asserter is there only for test purpose
-  ThreadCollisionWarner(AsserterBase* asserter = new DCheckAsserter())
+  explicit ThreadCollisionWarner(AsserterBase* asserter = new DCheckAsserter())
       : valid_thread_id_(0),
         counter_(0),
         asserter_(asserter) {}