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Introduces a formal split between a dialog model and the bubble in which
it's hosted. Refactors BookmarkBubbleView to use it.

Several DCHECKs are in place and TODOs that need to be resolved while
converting more dialogs to make this more flexible.

Bug: 1106422
Change-Id: Ied4baeaf3237f22f770562dc3161030e8640354d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2292700
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797105}