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This is responsible for:
- Mediating between the controller and WebUI layers.
- Managing the widget the WebUI interface lives in.
- Holding the web view the WebUI interface lives in. This is necessary
  because creating a renderer on demand caused unacceptable latency in
  the commander prototype.

Reviewer note: Please note that there are 5 separate paths to close
- Dismissed by controller via ViewModel
- Dismissed by browser via Hide
- Dismissed by WebUI via OnDismiss
- Window closed (possibly from parent window closing)
- Object destroyed
I'm pretty sure they're all safe, but this part definitely needs a
second pair of eyes.

Bug: 1014639
Change-Id: I422754923a0be3ce3e2b7b6a500add76f7e5ecd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364225Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800609}