The client code was using time.Now() (wall time) to determine whether
the cause of a non-nil error meant that a timeout had occured. But on
Windows, the clock used for timers (time.After, time.Sleep, etc) is
much more accurate than the time.Now clock, which doesn't update
often.

But it turns out that as of the recent https://golang.org/cl/32478 we
already have the answer available easily. It just wasn't in scope.

Instead of passing this information along by decorating the errors
(risky this late in Go 1.8, especially with #15935 unresolved), just
passing along the "didTimeout" func internally for now. We can remove
that later in Go 1.9 if we overhaul Transport errors.

Fixes #18287 (I hope)

Change-Id: Icbbfceaf02de6c7ed04fe37afa4ca16374b58f3c
Reviewed-on: https://go-review.googlesource.com/34381
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>