Updates Profile and Trace handlers to reject requests for durations >=
WriteTimeout.

Modifies go tool pprof to print the body of the http response when
status != 200.

Fixes #18755

Change-Id: I6faed21685693caf39f315f003039538114937b0
Reviewed-on: https://go-review.googlesource.com/35564Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>