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

cc/trees/layer_tree_host_common.h

Index: cc/trees/layer_tree_host_common.h
diff --git a/cc/trees/layer_tree_host_common.h b/cc/trees/layer_tree_host_common.h
index 00ef692bcf179555479a2ba46d2c5cc1372380c5..e50a51410e2b29c66cf960515abd2a9ede3c98fc 100644
--- a/cc/trees/layer_tree_host_common.h
+++ b/cc/trees/layer_tree_host_common.h
@@ -136,6 +136,8 @@ class CC_EXPORT LayerTreeHostCommon {
     // franctional scroll offset.
     gfx::Vector2d scroll_delta;
 
+    ScrollUpdateInfo();
+
     bool operator==(const ScrollUpdateInfo& other) const;
 
     void ToProtobuf(proto::ScrollUpdateInfo* proto) const;
@@ -147,6 +149,12 @@ struct CC_EXPORT ScrollAndScaleSet {
   ScrollAndScaleSet();
   ~ScrollAndScaleSet();
 
+  // The inner viewport scroll delta is kept separate since it's special.
+  // Because the inner (visual) viewport's maximum offset depends on the
+  // current page scale, the two must be committed at the same time to prevent
+  // clamping.
+  LayerTreeHostCommon::ScrollUpdateInfo inner_viewport_scroll;
+
   std::vector<LayerTreeHostCommon::ScrollUpdateInfo> scrolls;
   float page_scale_delta;
   gfx::Vector2dF elastic_overscroll_delta;