Add a way to display visual metrics results computed by browsertime

[mstange]

This is an alternative proposal to #2019 and will give some of the benefits with a much smaller scope.

Browsertime lets us run page load tests automatically. It also computes visual metrics for those page loads. And as of recently, it can also capture profiles for these page load runs.

It would be nice to embed the results of browsertime's visual metrics computations into the profile, and have those results displayed in the profiler.

For example, for the different scores (e.g. "Speed index with histogram matching", "Speed index with perceptual metrics", "contentfulness"), we could list those scores in the profile meta dropdown at the top.

We could also graph visual completeness over time. Browsertime would embed percentages with timestamps into the profile, and then we could draw a graph into a new primary track.

This proposal has a much smaller scope than #2019 because it will not require any visual metric computation in the profiler; the profiler will only present data that has been embedded into the profile by an earlier part of the pipeline. However, this also means that none of this work can be used for profiles that are captured using the Gecko profiler add-on. Which is fine, for now.

[mstange]

One way this data could be embedded into the profile would be using counters. I think this would let us reuse much of the work that was done for the memory track.

[mstange]

This was implemented in #2054.

[julienw]

It's broken by sitespeedio/browsertime#1746 though :-(