exp/cookiejar: make cookie sorting deterministic.

Re-enable TestUpdateAndDelete, TestExpiration, TestChromiumDomain and
TestChromiumDeletion on Windows.

Sorting of cookies with same path length and same creation
time is done by an additional seqNum field.
This makes the order in which cookies are returned in Cookies
deterministic, even if the system clock is manipulated or on
systems with a low-resolution clock.

The tests now use a synthetic time: This makes cookie testing
reliable in case of bogus system clocks and speeds up the
expiration tests.

R=nigeltao, alex.brainman, dave
CC=golang-dev
https://codereview.appspot.com/7323063

Committer: Nigel Tao <nigeltao@golang.org>