Draft charter for Web Performance Working Group

Based on Adrian's input and the current Web Applications charter, here
is a proposed charter for the Working Group. Some blanks need to be
filled so comments are welcome. I only listed WebTiming as a deliverable
for the moment. We could add others and/or make the deliverable more
vague to give us more leeway.

Once it gets more stable, I'll put it on the website somewhere.

Philippe