[tab_under] Report kDidTabUnder once per navigation

This metric was slightly broken, in that for clients without the
experiment enabled, we potentially reported kDidTabUnder on every
cross origin redirect that was "suspicious". This CL ensures it is
reported at most once. 

Bug: 661629
Change-Id: If919357b005762cea0e3d33edd589e90e1c9d1e2
Reviewed-on: https://chromium-review.googlesource.com/731775
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510689}
3 files changed