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Also fix a memory management issue that was latent beforehand,
use the |DeleteDelegate| pattern that the shelf widget is already using,
for the navigation widget, since a widget delegate must outlive its
widget.

This makes it more clear that all the shelf components (nav widget,
hotseat, status area, shelf widget) are now meant to play a similar
role in the class hierarchy.

This change isn't expected to have any impact on user-visible behavior.

Change-Id: I78688ec4f4fda7185dfcdf9e260c94f083d94923
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032188
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737214}