https://cs.chromium.org/chromium/src/ui/gfx/harfbuzz_font_skia.cc?type=cs&q=CreateHarfBuzzFont&sq=package:chromium&g=0&l=288

  // TODO(ckocagil): This shouldn't grow indefinitely. Maybe use base::MRUCache?
  static std::map<SkFontID, FaceCache> face_caches;


As far as I can tell this function is only called from the browser process so the set of fonts passed in is pretty limited, but there are many callers and the code is in ui/, so it's not unthinkable that it's going to be used from the renderer process, where webfonts means this cache could grow arbitrarily large over time.