Made layout viewport scroll updates from compositor work like ordinary layers.

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 490a1153d63f869362dbe06854dda6060a511d4d..b3deb42eae356e764c08abf5ee8135b5ef1d1713 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -71,6 +71,7 @@
 #include "core/layout/compositing/CompositedSelection.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
 #include "core/layout/svg/LayoutSVGRoot.h"
+#include "core/loader/DocumentLoader.h"
 #include "core/loader/FrameLoader.h"
 #include "core/loader/FrameLoaderClient.h"
 #include "core/page/AutoscrollController.h"
@@ -3394,6 +3395,11 @@ void FrameView::setScrollOffset(const DoublePoint& offset, ScrollType scrollType
     frame().loader().saveScrollState();
     frame().loader().client()->didChangeScrollOffset();
 
+    if (scrollType == CompositorScroll && m_frame->isMainFrame()) {
+        if (DocumentLoader* documentLoader = m_frame->loader().documentLoader())
+            documentLoader->initialScrollState().wasScrolledByUser = true;
+    }
+
     if (scrollType != AnchoringScroll)
         clearScrollAnchor();
 }