cmd/vendor/.../pprof: sync at rev 1ddc9e2

This includes changes in pprof to support
 - the new -diff_base flag
 - fix for a bug in handling of legacy Go heap profiles

Update #25096

Change-Id: I826ac9244f31cc2c4415388c44a0cbe77303e460
Reviewed-on: https://go-review.googlesource.com/115295


Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>