In order to tailor certain security checks to the caller that is attempting to
load a particular piece of content, we need to be able to attach an
appropriate triggering principal to the corresponding requests. Since most
HTML content is loaded based on attribute values, that means capturing the
subject principal of the caller who sets those attributes, which means making
it available to AfterSetAttr hooks.
MozReview-Commit-ID: BMDL2Uepg0X
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extra : rebase_source : 25e438c243700a9368c393e40e3a6002d968d6c8
