Adds AssistantProgressIndicator.

Used to indicate progress to the user.
This CL does not implement motion specs, to be added in a follow up.

See bug for demo.

Bug: b:111573788
Change-Id: I7d617cc4de6e7b85c7e9f053688738e49b424578
Reviewed-on: https://chromium-review.googlesource.com/1142499
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576594}
5 files changed