RFC: WebApp Timing (Rnd 2)

     Updated to incorporate the feedbacks. (Sorry for the long pause. I've
been in absence for the past couple weeks.)
  1) the window.pageTiming attribute should be available for each browsing
context, not only the top-level one.
  2) timing values are now in double/float so granularities finer than msec
are possible, even though not mandated yet.

cheers,
Zhiheng