Revert "Change height of Assistant caption bar."

This reverts commit 1f5d78a9.

Reason for revert: This CL is, in some cases, preventing Assistant
cards from rendering. Not sure why, but will revert until I can
find the root cause.

Original change's description:
> Change height of Assistant caption bar.
>
> Per change in spec, the Assistant caption bar should be 48dip tall.
> See bug for detailed spec.
>
> Bug: b:119498360
> Change-Id: I40cd7cd44a37b9ca99710dad9311b929373b3797
> Reviewed-on: https://chromium-review.googlesource.com/c/1336525
> Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
> Commit-Queue: David Black <dmblack@google.com>
> Cr-Commit-Position: refs/heads/master@{#608232}

TBR=xiaohuic@chromium.org,dmblack@google.com

Change-Id: I7c46f52c84c5b5db6f7e5a8f482d67100f50a911
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: b:119498360
Reviewed-on: https://chromium-review.googlesource.com/c/1338403Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#608558}

ash/assistant/ui/caption_bar.cc

@@ -13,6 +13,7 @@
 #include "ui/gfx/color_palette.h"
 #include "ui/gfx/paint_vector_icon.h"
 #include "ui/strings/grit/ui_strings.h"
+#include "ui/views/background.h"
 #include "ui/views/controls/button/image_button.h"
 #include "ui/views/layout/box_layout.h"
 #include "ui/views/widget/widget.h"
@@ -23,7 +24,7 @@ namespace {
 
 // Appearance.
 constexpr int kCaptionButtonSizeDip = 32;
-constexpr int kPreferredHeightDip = 48;
+constexpr int kPreferredHeightDip = 32;
 constexpr int kVectorIconSizeDip = 12;
 
 // CaptionButton ---------------------------------------------------------------