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So that we can set up necessary throttles and resource/request contexts
when Prefetch needs to create additional requests, e.g. for fetching
certificate for Signed HTTP Exchanges.

(The usage example of the throttles, resource_context and request_context_getter
would look like: https://chromium-review.googlesource.com/c/chromium/src/+/920565)

Bug: 803776
Change-Id: I71b1e7e8bb1178ecae89b4d9e37b60df658eba32
Reviewed-on: https://chromium-review.googlesource.com/923467Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537738}