[NavigationTiming] Reference check

   Per ACTION-51 <http://www.w3.org/2010/webperf/track/actions/51>, here is
the status summary of the references in NavigationTiming.

*Summary*: no invalid/obsolete reference found in the current spec and the
risk of they become invalid/obsolete in the future is low.

*Details*:
- Here are the major articles that the spec is referring to at this time,
along with their status:

   - WebIDL: Last Call since 09/27/2011
   - HTML5: Last Call since 05/25/2011
   - PerformanceTimeline: Last call since 9/1/2011
   - DOM 3 Core Specification: recommendation

- Most of the pointers to HTML5 refer to concepts without depending on how
they are processed. These references
have been pretty stable.
  * prompt to unload
  * unload
  * origin
  * HTTP redirects or equivalent
  * HTTP GET or equivalent
  * (relevant) application caches
  * fetch
  * history travesal
  * location.reload

- Two references are related to processing models:
   * fetchStart has a reference to step 13 of the navigation process when it
says "the time immediately before the user agent starts checking any
relevant application caches". The processing model did change but the
reference remains valid over time.
   * DOM-related attributes rely on the values of document.readyState, which
have been stable so far.

cheers,
Zhiheng