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This CL defines an interface with which //content embedders can satisfy
requests for "local" issuance of trust tokens
(https://github.com/wicg/trust-token-api), which are normally obtained
by querying a token issuer's HTTP server but may in some instances be
able to be provided by on-device entities, like system services or other
trusted applications.

Like the comment in trust_tokens.mojom discusses, we're adding this to
//content/public because the implementations will likely, at least in
part, vary by operating system and by specific browser brand or app.

Design doc for one implementation: http://go/clank-device-integrity
(internal only, sorry).

Fixed: 1130276
Change-Id: I577050aaae103288576e04e9f75330a16bce718b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493520
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820529}