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Also transitions the origin prober over to using the new probe result
enum so that it can be passed to the metrics collector.

ScopedRefPtr:
The metrics collector uses a scoped ref ptr to manage its lifetime.
For mainframe prefetching, the TabHelper has the only refptr to the
collector. But during NSP, the refptr is shared to every subresource
manager that is created.
If the next navigation does not go to a prefetched page, all subresource
managers are destroyed, as well as the TabHelper's CurrentPageLoad class
which destroys the metrics collector. Since there would not be any
cache reuse to record in the metrics, it can be safely destroyed then.
If the next navigation goes to a prefetched page, then all refptrs to
the metrics collector will be destroyed, except for the one in the
subresource manager associated with that page. This allows the
subresource manager to record cache reuse directly, without relying on
the TabHelper to make some lifetime guarantee about the metrics
collector.
Furthermore, this pattern keeps the cache reuse logic out of the
TabHelper when it is already isolated to the subresource manager.

Bug: 1136174
Change-Id: I4e0e52acb8f99a1490f67d98b44329f997e40720
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463544
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818670}