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Since holding space only supports a specific list of allowed volumes,
its welcome banner should only be shown on those volumes which the
feature supports.

Note that the holding space banner is also explicitly disallowed from
showing on Drive per UX request.

Bug: 1154347
Change-Id: I89e0c14ec9a6d1ec01700bd084e769d349243330
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594314
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837741}