1859326 - Serialize the marker subcategories

Status: NEW

(Core :: Gecko Profiler, enhancement, P3)

Description

[Nazım Can Altınova [:canova][:canaltinova on phabricator]]

Our current marker API lets us add a marker with a category and a sub category. We save this info into the profiler bufffer but we never read that in the end. Here's where we serialize this information:
https://searchfox.org/mozilla-central/rev/81f368dab93fff035ce7fcc376e16990e89dd5ec/mozglue/baseprofiler/public/BaseProfilerMarkersDetail.h#378-379

We should also serialize the sub category here so we can visualize it in our frontend.