Implements fetching URL-keyed hints on page navigation fetches.

This change adds functionality to the hints fetcher and manager to enable
fetching hints that are keyed by URL. Initially, only page navigation
fetches populate the URL in the hints fetch request.

Bug: 1036490
Change-Id: Iea65539122ebcbec0c395a090285551abb722f6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989981
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732143}