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New actions include:
- kShowContextMenu
- kShowPreviews
- kHidePreviews

This should tell us how discoverable the holding space pod context menu
is and how often users are enabling/disabling previews.

Bug: 1131266
Change-Id: I2b3d8c02afd6167ee1b516780ac29669e8566b4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531495
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826419}