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..b9019b7e707f7f56b3cbe61b28e85b6437403e94 100644
--- a/cc/trees/layer_tree_host_common.h
+++ b/cc/trees/layer_tree_host_common.h
@@ -147,6 +147,12 @@ 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;