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New HoldingSpaceClient APIs will be accessed from //ash to:
- Copy an item to clipboard (stubbed)
- Open an item in its folder (implemented)
- Pin an item (stubbed)
- Unpin an item (stubbed)

Stubbed APIs will be implemented in a follow up CL.

Bug: 1126274
Change-Id: Ie6790e935272d90064f278d8d916fd3b33445752
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399764Reviewed-by: David Black <dmblack@google.com>
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#805407}