HSTS priming changes the order of mixed-content blocking and HSTS
upgrades, and adds a priming request to check if a mixed-content load is
accesible over HTTPS and the server supports upgrading via the
Strict-Transport-Security header.
Every call site that uses AsyncOpen2 passes through the mixed-content
blocker, and has a LoadInfo. If the mixed-content blocker marks the load as
needing HSTS priming, nsHttpChannel will build and send an HSTS priming
request on the same URI with the scheme upgraded to HTTPS. If the server
allows the upgrade, then channel performs an internal redirect to the HTTPS URI,
otherwise use the result of mixed-content blocker to allow or block the
load.
nsISiteSecurityService adds an optional boolean out parameter to
determine if the HSTS state is already cached for negative assertions.
If the host has been probed within the previous 24 hours, no HSTS
priming check will be sent.
MozReview-Commit-ID: ES1JruCtDdX
