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This CL pulls the holding space tray icon out into a separate class.
Moving forward, the new HoldingSpaceTrayIcon will be modified to add
support for a content forward UI and accompanying motion spec. Changes
will be gated by a new feature flag (or param to the existing flag).

No changes should be visible to the user as a result of this CL.

Bug: 1142572
Change-Id: I21f85f37eeb595f7aa9ac3c79a171634b22e939b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2499376Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#821343}