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

cc/trees/property_tree.cc

Index: cc/trees/property_tree.cc
diff --git a/cc/trees/property_tree.cc b/cc/trees/property_tree.cc
index 795e337aa18b94cce584e717df8ef09f5449290f..808d6599ddb9717d31f53429035ad088ec76f222 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -1672,6 +1672,16 @@ bool ScrollTree::SetScrollOffset(int layer_id,
   return false;
 }
 
+void ScrollTree::AddUnappliedScrollOffsetDeltaToPendingTree(
+    int layer_id,
+    const gfx::ScrollOffset& scroll_offset_delta) {
+  DCHECK(!property_trees()->is_main_thread);
+  DCHECK(!property_trees()->is_active);
+
+  synced_scroll_offset(layer_id)->AddUnappliedDeltaToPendingBase(
+      scroll_offset_delta);
+}
+
 bool ScrollTree::UpdateScrollOffsetBaseForTesting(
     int layer_id,
     const gfx::ScrollOffset& offset) {