Re-enable UMA stats for predicted OOPIF process counts.
Re-enable UMA stats for predicted OOPIF process counts.

This was disabled in r225815 when the in-progress implementation
of out-of-process iframes would have corrupted the results, but
the values should be correct now.

Note that SiteIsolation.CurrentRendererProcessCount and
Memory.RendererProcessCount don't include subframe processes when
--site-per-process is enabled.  This happens to make the data
easier to interpret, since it matches the default process model
whether the user runs with --site-per-process or not.

BUG=248299

Review URL: https://codereview.chromium.org/1249843003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339942}

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