As discussed in  bug 884807  there is a path that currently creates a mojo::DataPipe on the service worker thread.  We might want to move this creation to a background task since creating the pipe involved synchronous I/O that can take milliseconds to complete.

While  bug 884807  is trying to reduce the number of paths that trigger the pipe creation, but there will always be some cases that need a pipe.  In those cases perhaps we could run a background task to create the pipe instead of doing it inline on the service worker thread.