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 af3638057b21ad9d63567520d83a252ac2a5eaff..e568382b95859eb54e25b1e3bb30958925be2482 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
 
   HostWindow* getHostWindow() 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.
@@ -382,7 +384,8 @@ class CORE_EXPORT PaintLayerScrollableArea final
   LayoutRect scrollIntoView(const LayoutRect&,
                             const ScrollAlignment& alignX,
                             const ScrollAlignment& alignY,
-                            ScrollType = ProgrammaticScroll) override;
+                            ScrollType = ProgrammaticScroll,
+                            bool isSmooth = false) override;
 
   // Returns true if scrollable area is in the FrameView's collection of
   // scrollable areas. This can only happen if we're scrollable, visible to hit