Fixed position layer counter-scroll with main thread scroll offset fractional part

cc/trees/layer_tree_host_common.cc

Index: cc/trees/layer_tree_host_common.cc
diff --git a/cc/trees/layer_tree_host_common.cc b/cc/trees/layer_tree_host_common.cc
index e12e272653ab7fa2ed335c41ed084711a0da16c0..4a14d02432ccf754fa1bef7c3e1d53441239d7f5 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -46,13 +46,15 @@ static void SortLayers(LayerImplList::iterator first,
 
 template <typename LayerType>
 static gfx::Vector2dF GetEffectiveScrollDelta(LayerType* layer) {
-  gfx::Vector2dF scroll_delta = layer->ScrollDelta();
+  gfx::Vector2dF scroll_delta =
+      layer->ScrollDelta() + layer->MainScrollOffsetFracitionalPart();

[aelias_OOO_until_Jul13, 2015/01/28 02:04:00]

Please add a comment explaining why the fractional part needs to be included
here.

[Yufeng Shen (Slow to review), 2015/01/28 22:28:09]

Done.

   // The scroll parent's scroll delta is the amount we've scrolled on the
   // compositor thread since the commit for this layer tree's source frame.
   // we last reported to the main thread. I.e., it's the discrepancy between
   // a scroll parent's scroll delta and offset, so we must add it here.
   if (layer->scroll_parent())
-    scroll_delta += layer->scroll_parent()->ScrollDelta();
+    scroll_delta += layer->scroll_parent()->ScrollDelta() +
+                    layer->scroll_parent()->MainScrollOffsetFracitionalPart();
   return scroll_delta;
 }