Initialize create_disposition to FILE_OPEN in sandbox

This is a follow-up fix for:
https://crrev.com/90b41f569db5cff7a8b8d170b81516dd006f57ba

R=wfh@chromium.org
BUG=438157

Review URL: https://codereview.chromium.org/890643002

Cr-Commit-Position: refs/heads/master@{#313738}

sandbox/win/src/filesystem_dispatcher.cc

@@ -145,7 +145,7 @@ bool FilesystemDispatcher::NtOpenFile(IPCInfo* ipc,
   const wchar_t* filename = name->c_str();
 
   uint32 broker = TRUE;
-  uint32 create_disposition = 0;
+  uint32 create_disposition = FILE_OPEN;
   CountedParameterSet<OpenFile> params;
   params[OpenFile::NAME] = ParamPickerMake(filename);
   params[OpenFile::ACCESS] = ParamPickerMake(desired_access);