Specs: Move the LayoutManager and RenderNode logic from Element to

sky/specs/runloop.md

 Sky's Run Loop
 ==============
 
 Sky's run loop consists of running the following, at 120Hz (each loop
 takes 8.333ms):
 
 1. Send scroll and resize events if necessary, limiting each handler
    to 1ms, and limiting the total time spent on these handlers to 1ms.
 
-2. Update as much of layout as possible; after 1ms, stop, leaving the
-   remaining nodes unprepared.
+2. Fire animation frame callbacks for up to 1ms. Each callback can run
+   for up to 1ms. Once 1ms has expired, throw a (catchable)
+   EDeadlineExceeded exception. If it's not caught, drop subsequent
+   callbacks.
 
-3. Update as much of paint as possible; after 1ms, stop, leaving the
-   remaining nodes unprepared.
+3. Spend up to 1ms to update the render tree, including calling
+   childAdded(), childRemoved(), and getLayoutManager() as needed.
+   Once 1ms has expired, throw a (catchable) EDeadlineExceeded
+   exception, leaving the render tree in whatever state it has
+   reached.
 
-4. Send frame to GPU.
+4. Update as much of layout as possible; after 1ms, throw a
+   (catchable) EDeadlineExceeded exception, leaving the remaining
+   nodes unprepared.
 
-5. Run pending tasks until the 8.333ms expires. Each task may only run
+5. Update as much of paint as possible; after 1ms, throw a (catchable)
+   EDeadlineExceeded exception, leaving any remaining nodes
+   unprepared.
+
+6. Send frame to GPU.
+
+7. Run pending tasks until the 8.333ms expires. Each task may only run
    for at most 1ms, after 1ms they get a (catchable) EDeadlineExceeded
    exception. While there are no pending tasks, sleep.
+   Tasks are thingsl like:
+    - timers
+    - updating the DOM in response to parsing
+    - input events
+    - mojo callbacks
 
 TODO(ianh): Update the timings above to have some relationship to
 reality.