Implement a getter returning whether the user has trust tokens

This CL adds a new method to Document, hasTrustToken(<issuer>),
returning whether the user has any trust tokens
(https://github.com/wicg/trust-token-api) for the token issuer origin
given by |issuer|. This works by querying a new Mojo interface provided
by the network context via BrowserInterfaceBinders.

More specifically, the method first checks if it can associate the
calling top-level context (in the sense of
TrustTokenStore::SetAssociation) with |issuer| and, if not, fails. For
this reason, the Mojo handle is bound by trusted code to the document's
top-level context.

New code:
- //content, NetworkContext: boilerplate to bind the Mojo interface
- blink::Document: add a new mojo remote and bind it; pack API calls
into mojom::HasTrustTokenAnswerer requests and unpack responses
- Blink scheduling: boilerplate to add a new TaskType (necessary in
order to use HeapMojoRemote)

correctly; WPTs exercise the interface end to end.

Test: unit tests check that the logic in the network service behaves
Bug: 1061116
Change-Id: I0f7835cac15b5af3582bb560e116511c601f3b36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2103082
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757487}