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This will make our terminology consistent with UX terminology in the
spec and should hopefully clear up some amiguity/confusion that may
have arisen due to using the term "item" in a context not directly
referring to a holding space model item.

Bug: 1142572
Change-Id: I59adbeebbd83f95c1bf377c7941f8b149d6f4448
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523059
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824967}