Do PPB_FileIO Query and Read in the plugin process.

chrome/nacl/nacl_ipc_adapter.cc

Index: chrome/nacl/nacl_ipc_adapter.cc
diff --git a/chrome/nacl/nacl_ipc_adapter.cc b/chrome/nacl/nacl_ipc_adapter.cc
index a25709908109bbfe947416768412ee60fc7af0a1..e94c761bfaf0ceb0a42ccb115a54188a2c911acb 100644
--- a/chrome/nacl/nacl_ipc_adapter.cc
+++ b/chrome/nacl/nacl_ipc_adapter.cc
@@ -435,6 +435,12 @@ bool NaClIPCAdapter::OnMessageReceived(const IPC::Message& msg) {
           break;
         }
         case ppapi::proxy::SerializedHandle::FILE:
+          // IMPORTANT: The NaClDescIoDescFromHandleAllocCtor function creates
+          // a NaClDesc that checks the flags before reading and writing. This
+          // is necessary since PPB_FileIO now sends a file descriptor which may
+          // have write capabilities, and we don't want the plugin to be able to
+          // write with it and so bypass quota checks, which still happen in the
+          // host.
           nacl_desc.reset(new NaClDescWrapper(NaClDescIoDescFromHandleAllocCtor(
 #if defined(OS_WIN)
               iter->descriptor(),