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

cc/trees/layer_tree_host_common.h

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_COMMON_H_
 #define CC_TREES_LAYER_TREE_HOST_COMMON_H_
 
 #include <stddef.h>
 
 #include <limits>
@@ skipping 129 matching lines @@
 
     void ToProtobuf(proto::ScrollUpdateInfo* proto) const;
     void FromProtobuf(const proto::ScrollUpdateInfo& proto);
   };
 };
 
 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;
   float top_controls_delta;
   std::vector<std::unique_ptr<SwapPromise>> swap_promises;
 
   bool EqualsForTesting(const ScrollAndScaleSet& other) const;
   void ToProtobuf(proto::ScrollAndScaleSet* proto) const;
   void FromProtobuf(const proto::ScrollAndScaleSet& proto);
 
@@ skipping 27 matching lines @@
     function(tree_impl->LayerById(id));
   }
 }
 
 CC_EXPORT PropertyTrees* GetPropertyTrees(Layer* layer);
 CC_EXPORT PropertyTrees* GetPropertyTrees(LayerImpl* layer);
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_COMMON_H_