Now that firefox has added StyleBench to their PGO training automation it makes sense to enable cross-lang PGO so that both rust code and cpp code can get profiled. Firefox's CSS engine Stylo is written in rust and StyleBench is written to benchmark CSS performance.
Unfortunately their build system doesn't support cross-lang PGO as part of the MOZ_PGO automated build flag and interally they have switched to using taskcluster to run the steps separately so there's a good chance MOZ_PGO will stop working at some point. Therefore do the PGO steps manually.
The stylebench score increases from 92.1 to 101 with cross-lang PGO and WebRender enabled.
Signed-off-by: Joey Riches <firstname.lastname@example.org>