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 500f6ec7580f58b2ffa128c55177bd000f9cc40d..d4c22b82bfb456dc13ba26e358e2ec10f2e88bed 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -1635,6 +1635,16 @@ bool ScrollTree::SetScrollOffset(int layer_id,
   return false;
 }
 
+void ScrollTree::AddUnappliedDeltaOnPendingTree(
+    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) {