[NavigationTiming] RFC: ticking navigationStart

  In last week's conf call, there was a discussion about the appropriate
spot to time navigationStart in case of a redirect.
To elicit feedback from more audience, here is a summary:

      |-------redirect---------|----------destionation----------|
     A                          B                                    C

  * Proposal 1: in case of a redirect, navigationStart points to the start
of the destination page, i.e., Point B in the above
    diagram.
    - navigationStart is mostly isolated from the effect of unloading the
previous page so it doesn't have be guarded by the
      same origin policy like redirectStart/End and unloadEventStart/End.
    - navigationStart will not be the starting point of user experience in
case of a redirect, but redirectStart.

  * Proposal 2: in case of a redirect, navigationStart points to the start
of the redirect (Point A), while the start of the
    destination page is denoted by fetchStart (Point B).
    - when navigate within the same origin, navigationStart is the starting
point of the user experience.
    - navigationStart will be protected by the same origin policy and is not
always available.

   Similar situation goes for unloadEventStart/End as well, i.e., should
they refers to the unloading of the page prior to
the redirection, or that of the page prior to the destination page (which
might not need to be unloaded at all).

   The current draft use the first proposal while IE and Chrome follow the
second.

cheers,
Zhiheng