GitLab

Adds a ToolbarCoordinator, ToolbarView, and ToolbarModelChangeProcessor
to display and manage the contextual suggestions toolbar.

Also introduces a PropertyObservable base class in the modelutil
package.

BUG=822943

Change-Id: I252e1c48afc8f053a5271cefa9f4a7142d816e32
Reviewed-on: https://chromium-review.googlesource.com/974289
Commit-Queue: Theresa <twellington@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545182}