cc/blimp: Add synchronization for scroll/scale state.

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 46eb212fc3868d2c19a281b3a4a0e76dc450bbdf..a11fed78d62f2dc4a8e398db46e8548ea56c2666 100644
--- a/cc/trees/layer_tree_host_common.h
+++ b/cc/trees/layer_tree_host_common.h
@@ -173,6 +173,21 @@ class CC_EXPORT LayerTreeHostCommon {
   };
 };
 
+// A container for the state that was reported to the main thread during
+// BeginMainFrame, but could not be applied/resolved on the main thread.
+struct CC_EXPORT ReflectedMainFrameState {
+  struct ScrollUpdate {
+    int layer_id = Layer::LayerIdLabels::INVALID_ID;
+    gfx::Vector2dF scroll_delta;
+  };
+
+  ReflectedMainFrameState();
+  ~ReflectedMainFrameState();
+
+  std::vector<ScrollUpdate> scrolls;
+  float page_scale_delta;
+};
+
 struct CC_EXPORT ScrollAndScaleSet {
   ScrollAndScaleSet();
   ~ScrollAndScaleSet();