Fixed Windows build.

src/extensions/gc-extension.h

 // Copyright 2010 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 27 matching lines @@
   explicit GCExtension(const char* fun_name)
       : v8::Extension("v8/gc",
                       BuildSource(buffer_, sizeof(buffer_), fun_name)) {}
   virtual v8::Handle<v8::FunctionTemplate> GetNativeFunctionTemplate(
       v8::Isolate* isolate,
       v8::Handle<v8::String> name);
   static void GC(const v8::FunctionCallbackInfo<v8::Value>& args);
 
  private:
   static const char* BuildSource(char* buf, size_t size, const char* fun_name) {
-    OS::SNPrintF(Vector<char>(buf, size), "native function %s();", fun_name);
+    OS::SNPrintF(Vector<char>(buf, static_cast<int>(size)),
+                 "native function %s();", fun_name);
     return buf;
   }
 
   char buffer_[50];
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_EXTENSIONS_GC_EXTENSION_H_