Implement Element.scrollIntoView for scroll-behavior: smooth.

third_party/WebKit/Source/platform/scroll/ScrollableArea.h

Index: third_party/WebKit/Source/platform/scroll/ScrollableArea.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
index 266e8048a3ea6b78a75b4b005e13d20120d74627..68d2579671eddf3506b4d3a63faa03e20cd71462 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
@@ -50,6 +50,7 @@ class LayoutBox;
 class LayoutObject;
 class PaintLayer;
 class ProgrammaticScrollAnimator;
+class ProgrammaticScrollCoordinator;
 struct ScrollAlignment;
 class ScrollAnchor;
 class ScrollAnimatorBase;
@@ -80,6 +81,11 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin,
   // coordinate space.
   virtual HostWindow* getHostWindow() const { return 0; }
 
+  virtual ProgrammaticScrollCoordinator* getProgrammaticScrollCoordinator()

[bokan, 2017/03/28 16:29:53]

I don't think this should be part of this class' interface. As mentioned above,
Page is probably a better place and I think everything has access to Page.

[sunyunjia, 2017/03/31 15:42:45]

When each scrollableArea finishes its scroll, ProgrammaticScrollAnimator(PSA)
will notify SmoothScrollSequencer so that the next scrollableArea can start
scroll (see PSA in this patch). Unfortunately, neither of PSA and ScrollableArea
has access to Page. In fact, implementations of
ScrollableArea(PaintLayerScrollableArea, FrameView, RootFrameViewport) have
different path to access page. So I guess the best way to access
SmoothScrollSequencer from PSA is through ScrollableArea's interface? 

[bokan, 2017/03/31 20:30:46]

Ah, yes...ScrollableArea is in platform. It's a bit verbose but
ScrollableArea->layoutBox()->document().page() should do the trick. You'll have
to nullcheck layoutBox but if it doesn't have a layoutbox it wont be scrollable
so you can just abort. I'd add a private helper where you need it to get the
Page this way.

+      const {
+    return 0;
+  }
+
   virtual ScrollResult userScroll(ScrollGranularity, const ScrollOffset&);
 
   virtual void setScrollOffset(const ScrollOffset&,
@@ -102,7 +108,8 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin,
   virtual LayoutRect scrollIntoView(const LayoutRect& rectInContent,
                                     const ScrollAlignment& alignX,
                                     const ScrollAlignment& alignY,
-                                    ScrollType = ProgrammaticScroll);
+                                    ScrollType = ProgrammaticScroll,
+                                    bool isSmooth = false);
 
   static bool scrollBehaviorFromString(const String&, ScrollBehavior&);