Enable Assistant motion spec.

Enables the current motion spec implementation.
Also fixed a painting bug.

Follow up CL will remove the bool and dead code once these changes have
been more heavily tested in the wild.

Bug: b:112329634, b:112330123
Change-Id: Ibc6cf664d96125b6ca0fd322bde560f69a611f26
Reviewed-on: https://chromium-review.googlesource.com/1166293Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#581426}

ash/assistant/ui/assistant_ui_constants.h

@@ -32,7 +32,7 @@ namespace ui {
 // TODO(dmblack): Remove when enabling. This is used to gate aspects of the
 // motion spec which are being developed in pieces which should be enabled
 // together.
-constexpr bool kIsMotionSpecEnabled = false;
+constexpr bool kIsMotionSpecEnabled = true;
 
 // Returns the default font list for Assistant UI.
 const gfx::FontList& GetDefaultFontList();

ash/assistant/ui/main_stage/assistant_main_stage.cc

@@ -602,8 +602,8 @@ void AssistantMainStage::UpdateTopPadding() {
       views::CreateEmptyBorder(top_padding, 0, 0, 0));
 
   // Force a layout/paint pass.
-  content_layout_container_->Layout();
-  content_layout_container_->SchedulePaint();
+  ui_element_container_->Layout();
+  ui_element_container_->SchedulePaint();
 
   // Apply top padding to the overlay layout container by applying an empty
   // border to its children.