Fix presubmit error for recent revision.

src/code-stubs.h

Index: src/code-stubs.h
diff --git a/src/code-stubs.h b/src/code-stubs.h
index 63e292a51f60ac46de59e266181317a0d6c9ee9c..3295f015c6edca235b715974f36c96553542596c 100644
--- a/src/code-stubs.h
+++ b/src/code-stubs.h
@@ -386,7 +386,7 @@ class FastCloneShallowObjectStub : public CodeStub {
   // Maximum number of properties in copied object.
   static const int kMaximumClonedProperties = 6;
 
-  FastCloneShallowObjectStub(int length) : length_(length) {
+  explicit FastCloneShallowObjectStub(int length) : length_(length) {
     ASSERT_GE(length_, 0);
     ASSERT_LE(length_, kMaximumClonedProperties);
   }