[AnimationRequestFrame] Initial editor's draft of AnimationRequestFrame spec available

I've uploaded a first editor's draft of the AnimationRequestFrame spec to
http://dvcs.w3.org/hg/webperf/raw-file/tip/specs/AnimationRequestFrame/Overview.html
(current
version is
http://dvcs.w3.org/hg/webperf/raw-file/02822501c985/specs/AnimationRequestFrame/Overview.html).
 We have a bit of work to do but this document should provide a useful
starting point.

Here's a few of the known issues with this text:
- "getting the next sample time" algorithm is undefined
- The specifics of what it means for a user agent to "continually sample all
animations" is undefined.  We'll need to add some text here indicating the
user agent should update animations taking into account the page's
visibility with some accommodation for browser-level throttling and flow
control.

Additionally, we've experimented in Chrome with adding an element parameter
to requestAnimationFrame in order to do more specific visibility testing.  I
think this functionality is useful but it will require some refining to come
up with a consistent and well-defined description of what this means.  For
now, I've left this parameter out of the spec.

- James