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This CL enacts the "class splitting refactor"
(//docs/ui/views/class_splitting.md) on ColorChooserView, breaking it
into:

* A controller class called ColorChooser, whose lifetime is fully
  controlled by the client, and which is safe to call at any point in
  the lifecycle of the matching Widget/Views/etc
* Two helper objects (instances of WidgetDelegate and View respectively)
  whose lifetimes are fully controlled by Views

This CL serves as a prototype for how the class splitting refactor looks
when applied to production code.

Instead, have it use only the public API on WidgetDelegate.

Bug: 1075649
Change-Id: I84655a9c204a9f5f88767ac792ce1abc846feda0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622995
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846358}