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This is a preexisting metric capturing the way the bookmark icon was
activated that we want to continue to capture when read later is
enabled.

Bug: 1163938, 1131547
Change-Id: I0c397d7f534c6c52726978d3a85c9fa2a046a6a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616418
Commit-Queue: Caroline Rising <corising@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841546}