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Previously we were only recording the time of the user's first pin to
holding space. Now we will record the time when the first item of any
type was added to holding space.

This should help determine how discoverable the feature is.

Bug: 1131266
Change-Id: Iacee3d00345fa2860b34d7695d92bf3e0bf91485
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530751Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#827033}