refactor(clean): share Chromium Default cache cleanup

Antigravity and Chrome DevTools MCP both wipe the same five
regenerable paths under a Chromium Default profile (Cache, Code Cache,
GPUCache, DawnGraphiteCache, DawnWebGPUCache). Extract them into a
single clean_chromium_default_caches helper so future Chromium-based
profile cleaners don't have to copy-paste the list.

DevTools MCP still cleans its extra DawnCache, GrShaderCache, and
top-level GraphiteDawnCache paths separately. Set of removed paths
is identical before and after.