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Per request from UI/UX folks, the Assistant caption bar should have
horizontal insets of 8dip to give more separation between the buttons
and container edge.

See bug for screenshots.

Bug: b:115635716
Change-Id: I2edb54e50ea63bbc7a50dd15d1c962c7aa14f7a8
Reviewed-on: https://chromium-review.googlesource.com/1256363
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595945}