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When I implemented SignedExchangeDevToolsProxy (crrev.com/c/1025492), we need to
use callback to get the frame_tree_node_id. It was because when Network Service
is not enabled the ID is not available while handling prefetch requests on the
IO thread.

But now, we don't need to support non-NetworkService mode (crbug.com/934009).
So we can stop using callback.

Bug: 934009
Change-Id: Idf5bfa1b59ce3499b8e9f27659a044ec52765dd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863064
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706367}