GitLab

Resets current holding space model before calling holding space
detached, so HoldingSpaceController::Get()->model() can be used
test whether a model is currently attached from the observer method.

Also, updates HoldingSpaceTray to cancel previews update when previews
icon gets hidden.

BUG=1164145

Change-Id: I1f0b50cc52c5b348b0e034e65751b9a0047bd096
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618093Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841691}