cmd/trace: SWEEP events' swept/reclaimed bytes info is not emitted [1.11 backport]

[gopherbot]

@aclements requested issue #27711 to be considered for backport to the next 1.11 minor release.

@gopherbot, please backport to 1.11: this is a regression from Go 1.10.

[gopherbot]

Change https://golang.org/cl/146698 mentions this issue: [release-branch.go1.11] cmd/trace: don't drop sweep slice details

[gopherbot]

Closed by merging eb46d15 to release-branch.go1.11.