Win32 build fix

TBR=rossberg@chromium.org

BUG=

Review URL: https://codereview.chromium.org/13140009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14093 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/runtime.cc

@@ -801,7 +801,8 @@ static void ArrayBufferWeakCallback(v8::Isolate* external_isolate,
 
   size_t allocated_length = ArrayBufferAllocatedLength(
       isolate, JSArrayBuffer::cast(*internal_object));
-  isolate->heap()->AdjustAmountOfExternalAllocatedMemory(-allocated_length);
+  isolate->heap()->AdjustAmountOfExternalAllocatedMemory(
+      -static_cast<intptr_t>(allocated_length));
   if (data != NULL)
     free(data);
   object.Dispose(external_isolate);