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This adds variants of PageLoad.Clients.MultiTabLoading.* metrics that
are recorded only when there are >=2 (or >=5) inflight loading tabs.

Bug: 792759
Change-Id: Ic1c8711d6424752dd80685b7740ec12fcc1c885e
Reviewed-on: https://chromium-review.googlesource.com/826706Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524329}