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This adds initial support for doing full prefetching of subresources,
beyond preconnecting to their hosts.

The code is gated behind a feature flag kLoadingPredictorPrefetch.

The PrefetchManager logic is strongly based on the PreconnectManager.
But splitting them into two separate classes seemed cleaner overall.

Bug: 1092329
Change-Id: I26b244105d59e8765fc24c109ec165b7def39f89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2235192
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780204}