split view: Set the tween type of the white bar animation to EASE_IN.

For the start and end of dragging the split screen divider, the
specification indicates the tween type of the white handle animation as
fast out and slow in, instead of fast in and slow out.

Test: manual
Bug: 929972
Change-Id: I41f61dd4bed4e94e59e9ed1cb310c95d8bad6e7d
Reviewed-on: https://chromium-review.googlesource.com/c/1460308Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630345}

ash/wm/splitview/split_view_divider.cc

@@ -23,6 +23,7 @@
 #include "ui/compositor/scoped_layer_animation_settings.h"
 #include "ui/gfx/animation/animation_delegate.h"
 #include "ui/gfx/animation/slide_animation.h"
+#include "ui/gfx/animation/tween.h"
 #include "ui/gfx/canvas.h"
 #include "ui/views/view.h"
 #include "ui/views/view_targeter_delegate.h"
@@ -125,6 +126,7 @@ class DividerView : public views::View,
     SetEventTargeter(
         std::unique_ptr<views::ViewTargeter>(new views::ViewTargeter(this)));
     white_bar_animation_.SetSlideDuration(kWhiteBarBoundsChangeDurationMs);
+    white_bar_animation_.SetTweenType(gfx::Tween::EASE_IN);
   }
   ~DividerView() override { white_bar_animation_.Stop(); }