Files services library: Add an "output stream" file impl helper class.

apps/moterm/moterm_driver.h

Index: apps/moterm/moterm_driver.h
diff --git a/apps/moterm/moterm_driver.h b/apps/moterm/moterm_driver.h
index dd5a8bc54c9b530596acb327bd5ad7174edd8c4b..57a2ab2565c2176e837411d747bc989b05c72925 100644
--- a/apps/moterm/moterm_driver.h
+++ b/apps/moterm/moterm_driver.h
@@ -39,8 +39,8 @@ class MotermDriver : public mojo::files::File {
     // but not verified.
     virtual void OnDataReceived(const void* bytes, size_t num_bytes) = 0;
 
-    // Called when the terminal "file" is closed (via |Close()|. (The client may
-    // optionally call |Detach()| in response.)
+    // Called when the terminal "file" is closed (via |Close()|). (The client
+    // may optionally call |Detach()| in response.)
     virtual void OnClosed() = 0;
 
     // Called when this object is destroyed (which will happen if the other end