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This change introduces a new UKM for counting navigation redirects in
the pop up tracker. Redirects are commonly used for navigaing abusive
pop-ups to ad content from elements not tagged as ads by the ad
tagging framework. Using redirect counts as a new filter will help
us refine our abusive popup identification.

BUG=1044601

Change-Id: I68ad28a1b2965a8c1e844ec06eb42b487c6ef717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438669Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Justin Miron <justinmiron@google.com>
Cr-Commit-Position: refs/heads/master@{#820261}