Fix build on arm/x64 after r17840.

src/v8conversions.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 66 matching lines @@
       return true;
     } else {
       return false;
     }
   }
 }
 
 // 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;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_V8CONVERSIONS_H_