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Now that [SecureContext] works with interfaces, resolve some TODOs and
align the implementation of NavigationPreloadManager with the spec.

Change-Id: I6a8d156182112ee972eb4720e7fa478e784e73bc
Reviewed-on: https://chromium-review.googlesource.com/802620Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520982}