Memory track could show heat map for number of (de)allocation calls

Currently the memory track only shows the amount of memory (relative to the start of the process), as captured in the `"count"` field of memory counters.

But Firefox also provides a `"number"` field, which is the number of time an allocation/deallocation function was called.

This could be useful to show, to indicate time ranges where lots of memory operations happened, regardless of the number of bytes involved.
It could be a sort of heat map, showing shades of orange along the existing memory graph.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-448)