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/1336525Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#608232}

ash/assistant/ui/caption_bar.cc

@@ -13,7 +13,6 @@
 #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"
@@ -24,7 +23,7 @@ namespace {
 
 // Appearance.
 constexpr int kCaptionButtonSizeDip = 32;
-constexpr int kPreferredHeightDip = 32;
+constexpr int kPreferredHeightDip = 48;
 constexpr int kVectorIconSizeDip = 12;
 
 // CaptionButton ---------------------------------------------------------------