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The new UMA should help evaluate how often the spare is missing (e.g.
because of low process limit on low-memory devices), mismatched (e.g.
when simultaneously navigating in 2 profiles) or taken.  The spare can
only help improve performance of cross-site navigations if it is
actually taken.

Bug: 808114
Change-Id: Ic37cacf014de73f9adefc0407e60b4cfa246028b
Reviewed-on: https://chromium-review.googlesource.com/1012510
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551026}