[NavigationTiming]Working draft updated

Hey, folks,

   Just a heads-up that the
NavigationTiming<https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/NavigationTiming/Overview.html>working
draft is now updated with some recent discussions:
   * having requestStart to include cache lookup time and
the corresponding processing model change
   * define "time" to simplify description
   * specifically mention corner cases like multiple readiness changes.
   * add a section for garbage collection
   * other minor typo fixes


https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/NavigationTiming/Overview.html

cheers,
Zhiheng