Force signed zero double.

R=svenpanne@chromium.org
BUG=v8:2239
TEST=

Review URL: https://chromiumcodereview.appspot.com/10821088

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12219 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/conversions.h

@@ -53,7 +53,7 @@ inline bool isDigit(int x, int radix) {
 
 
 inline double SignedZero(bool negative) {
-  return negative ? -0.0 : 0.0;
+  return negative ? BitCast<double, uint64_t>(Double::kSignMask) : 0.0;
 }