runtime/pprof: scale mutex profile with sampling rate

pprof expects the samples are scaled and reflects unsampled numbers.
The legacy profile parser uses the sampling period in the output
and multiplies all values with the period.

https://github.com/google/pprof/blob/0138a3cd6dad6f94495ba0b5c3a5c124f04ae011/profile/legacy_profile.go#L815

Apply the same scaling when we output the mutex profile
in the pprof proto format.

Block profile shares the same code, but how to infer unsampled
values is unclear. Legacy profile parser doesn't do anything special
so we do nothing for block profile here.

Tested by checking the profiles reported with debug=0 (proto format)
are similar to the profiles computed from legacy format profile
when the profile rate is a non-trivial number (e.g. 2) manually.

Change-Id: Iaa33f92051deed67d8be43ddffc7c1016db566ca
Reviewed-on: https://go-review.googlesource.com/89295


Reviewed-by: Peter Weinberger <pjw@google.com>