Fix warnings on Win64.

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index e9f67ef430bdfa101264ef61b09c0528b1dd76b3..4491962163d99dc524136ce98fe1da9358d01703 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -98,7 +98,7 @@ static List<char*>* delete_these_non_arrays_on_tear_down = NULL;
 
 
 NativesExternalStringResource::NativesExternalStringResource(const char* source)
-    : data_(source), length_(strlen(source)) {
+    : data_(source), length_(StrLength(source)) {
   if (delete_these_non_arrays_on_tear_down == NULL) {
     delete_these_non_arrays_on_tear_down = new List<char*>(2);
   }