Implement Element.scrollIntoView for scroll-behavior: smooth.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
index c39682762dc7f3caf4bce683e59babe6f446b54e..ff607f8f2d4968146b5b51fa7fa44cebce964f3f 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
@@ -240,6 +240,8 @@ class CORE_EXPORT PaintLayerScrollableArea final
 
   PlatformChromeClient* GetChromeClient() const override;
 
+  SmoothScrollSequencer* GetSmoothScrollSequencer() const override;
+
   // For composited scrolling, we allocate an extra GraphicsLayer to hold
   // onto the scrolling content. The layer can be shifted on the GPU and
   // composited at little cost.
@@ -392,6 +394,7 @@ class CORE_EXPORT PaintLayerScrollableArea final
   LayoutRect ScrollIntoView(const LayoutRect&,
                             const ScrollAlignment& align_x,
                             const ScrollAlignment& align_y,
+                            bool is_smooth,
                             ScrollType = kProgrammaticScroll) override;
 
   // Returns true if scrollable area is in the FrameView's collection of