net/http/pprof: use Request.Context, not the deprecated CloseNotifier

Prior to this commit, the profiling code had a sleep() function that
waits and unblocks on either time.After() or a channel provided by an
http.CloseNotifier derived from a supplied http.ResponseWriter.

According to the documentation, http.CloseNotifier is deprecated:

  Deprecated: the CloseNotifier interface predates Go's context package.
  New code should use Request.Context instead.

This patch does just that -- sleep() now takes an *http.Request and uses
http.Request.Context() to signal when a request has been cancelled.

Change-Id: I98702314addf494f5743a4f99172dc607389dbb8
GitHub-Last-Rev: c1e37a03ca28417ed5833618d3eeddb2eecccd09
GitHub-Pull-Request: golang/go#41756
Reviewed-on: https://go-review.googlesource.com/c/go/+/259157


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>