Update documentation of Performance.timing/navigation

Update documentation of Performance.timing/navigation

The specs have now been updated:
https://github.com/w3c/navigation-timing/issues/22
https://github.com/w3c/navigation-timing/issues/28
https://github.com/w3c/navigation-timing/issues/29

BUG=460722
R=ksakamoto@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=199361

[philipj_slow, 2015-07-22 09:31:18]

The CQ bit was checked by philipj@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2015-07-22 09:32:07]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1245343003/1

[commit-bot: I haz the power, 2015-07-22 10:52:41]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-22 10:52:41]

Dry run: This issue passed the CQ dry run.

[Kunihiko Sakamoto, 2015-07-23 01:22:17]

lgtm

[philipj_slow, 2015-07-23 07:51:58]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2015-07-23 07:52:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1245343003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1245343003/1

[commit-bot: I haz the power, 2015-07-23 07:54:53]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=199361