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This CL replaces the pick_one Selector filter by the nth_match filter,
that matches the nth element of the elements currently matching.

See [1] for more background.

[1] https://docs.google.com/document/d/167LaI_WQNr31wBuO3DNlCPD2sx3If2GOHpPsT4atXIk

Change-Id: I3024239d256201fe604c0c72b4985a9de3d45201
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517535
Commit-Queue: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Cr-Commit-Position: refs/heads/master@{#824854}