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This change:
1) Adds a new function, chrome::ShowDeviceChooserDialog, that displays a
   device chooser bypassing the BubbleUi framework
2) Has Browser use this new function rather than BubbleUi to show the
   bluetooth device choosers

Note that ShowDeviceChooserDialog returns a OnceClosure, which the client
can call to close the dialog. It does not return a pointer to the dialog
since the dialog is self-owning. Previously, this code dealt with that
by passing around BubbleReferences, which have weak pointer semantics;
returning a OnceClosure (which internally has a WeakPtr to the UI)
allows the client code to close the bubble when it wants to without adding
new lifetime issues.

Remaining uses of ChooserBubbleDelegate directly:
* Web USB chooser
* HID chooser
* Serial chooser

These use similar design patterns, and will be migrated in follow-up
changes.

Bug: 496955
Change-Id: I1b45c1cfdba6e0eaee57ef61d91da294832a0888
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2102947
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750540}