[NavigationTiming] a couple questions

    Jonas brought up a couple questions about the current draft and we
decide to
have them discussed here.

- "One thing that is a bit unclear to me is the navigationStart
attribute. In one of the two figures it is indicated to be available
on same-origin only navigation. But in the other figure, and in the
description of the attribute itself, it seems like it's available on
cross-origin navigation too."

   The first figure doesn't include redirect, in which case navigationStart
is available all the time regardless the origins. Meanwhile,
unloadEventStart
and unloadEventEnd only reveal under the same origin. (They should have
been underlined in the first figure. I will fix it in the next update.)

   The recent discussion on SOP has been very educating. :-)

 - "Also, to be clear, this allows a page to know if a user arrived at the
page using back/forward, by using reload, or by navigating to the page
through other means?"

   yes, window.performance.navigation.type.

- "Also, an issue that isn't related to privacy. This introduces a new
global attribute called 'performance', right? This seems somewhat
scary as it is quite a generic name, and will break any web pages that
uses a global variable with that name."

  hmm, any other name to suggest?

cheers,
Zhiheng