Fixed horizontal alignment in Dialog Plate.

Previously, the animated voice input toggle was slightly off center:
https://screenshot.googleplex.com/tODAXyEw64F

Bug: 924624
Change-Id: I30b230a22afa152a4081aa9f25347a696a002179
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506381
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638325}

ash/app_list/views/assistant/dialog_plate.cc

@@ -335,10 +335,20 @@ void DialogPlate::InitVoiceLayoutContainer() {
   layout_manager->set_cross_axis_alignment(
       views::BoxLayout::CrossAxisAlignment::CROSS_AXIS_ALIGNMENT_CENTER);
 
+  // Offset.
+  // To make the |animated_voice_input_toggle_| horizontally centered in the
+  // dialog plate we need to offset by the difference in width between the
+  // |molecule_icon_| and the |keyboard_input_toggle_|.
+  constexpr int difference =
+      /*keyboard_input_toggle_width=*/kButtonSizeDip -
+      /*molecule_icon_width=*/kIconSizeDip;
+  views::View* offset = new views::View();
+  offset->SetPreferredSize(gfx::Size(difference, 1));
+  voice_layout_container_->AddChildView(offset);
+
   // Spacer.
   views::View* spacer = new views::View();
   voice_layout_container_->AddChildView(spacer);
-
   layout_manager->SetFlexForView(spacer, 1);
 
   // Animated voice input toggle.
@@ -351,19 +361,8 @@ void DialogPlate::InitVoiceLayoutContainer() {
   // Spacer.
   spacer = new views::View();
   voice_layout_container_->AddChildView(spacer);
-
   layout_manager->SetFlexForView(spacer, 1);
 
-  // Spacer.
-  // To make the mic icon in the center of the |assistant_page_view_|
-  // compensate for the width of molecule icon and keyboard input toggle.
-  spacer = new views::View();
-  constexpr int spacing =
-      /*molecule_icon_width=*/kIconSizeDip -
-      /*keyboard_input_toggle_width*/ kButtonSizeDip;
-  spacer->SetPreferredSize(gfx::Size(spacing, 1));
-  voice_layout_container_->AddChildView(spacer);
-
   // Keyboard input toggle.
   keyboard_input_toggle_ = ash::AssistantButton::Create(
       this, ash::kKeyboardIcon, kButtonSizeDip, kIconSizeDip,