Search the sample types in the profile being processed to map
sample type options to indices in the profile sample type array.

Previously these were hardcoded, which caused issues when the
sample types for a profile type changed. For instance, this was
triggered by the native generation of profiles in profile.proto
format.

This fixes #18230. A similar mechanism already exists on the upstream
pprof.

Change-Id: I945d8d842a0c2ca14299dabefe83124746ecd7e2
Reviewed-on: https://go-review.googlesource.com/34382Reviewed-by: Michael Matloob <matloob@golang.org>