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A new UKM event, PreviewsResourceLoadingHints, is being added. This
tracks the usage of the patterns associated with a resource loading
hints based preview and the total number of resource requests blocked
at each resource load priority.

The UKM is recorded after the URL's document finished loading and uses
the NavigationID from the browser's NavigationHandle as the source id.

Bug: 891453, 884815
Change-Id: I429a6ef5e6222d7f569c8d2eb5539d815af6ff02
Reviewed-on: https://chromium-review.googlesource.com/c/1307057
Commit-Queue: Jered Gray <jegray@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605034}