Add insets to Assistant caption bar.

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}

ash/assistant/ui/caption_bar.cc

@@ -87,8 +87,8 @@ void CaptionBar::SetButtonVisible(CaptionButtonId id, bool visible) {
 void CaptionBar::InitLayout() {
   views::BoxLayout* layout_manager =
       SetLayoutManager(std::make_unique<views::BoxLayout>(
-          views::BoxLayout::Orientation::kHorizontal, gfx::Insets(),
-          kSpacingDip));
+          views::BoxLayout::Orientation::kHorizontal,
+          gfx::Insets(0, kSpacingDip), kSpacingDip));
 
   layout_manager->set_cross_axis_alignment(
       views::BoxLayout::CrossAxisAlignment::CROSS_AXIS_ALIGNMENT_CENTER);