Fix presubmit error introduced by r9932.

R=erik.corry@gmail.com
BUG=
TEST=

Review URL: http://codereview.chromium.org/8511010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9934 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap.h

@@ -282,7 +282,7 @@ class HeapDebugUtils;
 // by it's size to avoid dereferencing a map pointer for scanning.
 class PromotionQueue {
  public:
-  PromotionQueue(Heap* heap)
+  explicit PromotionQueue(Heap* heap)
       : front_(NULL),
         rear_(NULL),
         limit_(NULL),