Fix build on arm/x64 after r17840.

src/v8conversions.h

Index: src/v8conversions.h
diff --git a/src/v8conversions.h b/src/v8conversions.h
index b449c8884216f0353bca9aaa8ba92aaeae9c2012..68107de97a21a87c5c2d4e490d5b0248dff96f06 100644
--- a/src/v8conversions.h
+++ b/src/v8conversions.h
@@ -84,7 +84,7 @@ inline bool TryNumberToSize(Isolate* isolate,
 // Converts a number into size_t.
 inline size_t NumberToSize(Isolate* isolate,
                            Object* number) {
-  size_t result;
+  size_t result = 0;
   bool is_valid = TryNumberToSize(isolate, number, &result);
   CHECK(is_valid);
   return result;