Fixed Windows build.

src/extensions/gc-extension.h

Index: src/extensions/gc-extension.h
diff --git a/src/extensions/gc-extension.h b/src/extensions/gc-extension.h
index adc79fe3398b3a8e5c2cc486bb6f68b74f923492..105c5ad5b72b4f01ed3712c2eec2613fc218449f 100644
--- a/src/extensions/gc-extension.h
+++ b/src/extensions/gc-extension.h
@@ -45,7 +45,8 @@ class GCExtension : public v8::Extension {
 
  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;
   }