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This change moves the ExtensionInstalledBubble's model, which was
previously a collection of anonymous functions in the Views implementation,
into //cbui/extensions as a separate class with unit tests. It also changes
the Views implementation to use this new class.

Change-Id: I3e379db88fcd4d40014c74f14c67b155dde3ac63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007508
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739021}