The trace command computes IO, Schedule, Block, and Syscall profiles
by following the unblocking links in the execution trace and summing
up the duration.  This change offers variations of those profiles
that include only selected goroutine types. The id parameter takes the
goroutine type - i.e. pc of the goroutine.

The output is available from the /goroutine view. So, users can see
where the goroutines of interest typically block.

Also, these profiles are available for download so users can use
pprof or other tools to interpret the output. This change adds links
for download of global profile in the main page.

Change-Id: I35699252056d164e60de282b0406caf96d629c85
Reviewed-on: https://go-review.googlesource.com/75710Reviewed-by: Sameer Ajmani <sameer@golang.org>