cmd/trace: list and link to worst mutator utilization windows

This adds the ability to click a point on the MMU graph to show a list
of the worst 10 mutator utilization windows of the selected size. This
list in turn links to the trace viewer to drill down on specifically
what happened in each specific window.

Change-Id: Ic1b72d8b37fbf2212211c513cf36b34788b30133
Reviewed-on: https://go-review.googlesource.com/c/60795


Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>