The preload list is static, immutable data compiled into LibHTTP.
ResourceLoader consults it in-process at the fetch layer before
falling back to the dynamic, per-profile store in the browser
process, so a preloaded host is upgraded without a synchronous IPC.
HSTSStore stays the dynamic store only; the preload list cannot be
unset by a max-age=0 response because it is consulted first, before
the store is ever queried.
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