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

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 27c7cb429e6c00a2681847ba4d1c60bda05ce399..29fd1a7f44546f79f022d2228ea735282b19d673 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -2078,4 +2078,37 @@ void LayerTreeImpl::ResetAllChangeTracking() {
   property_trees_.ResetAllChangeTracking();
 }
 
+void LayerTreeImpl::SetReflectedMainFrameState(
+    std::unique_ptr<ReflectedMainFrameState> reflected_main_frame_state) {
+  DCHECK(!IsActiveTree());
+  reflected_main_frame_state_ = std::move(reflected_main_frame_state);
+}
+
+void LayerTreeImpl::ApplyReflectedMainFrameState() {
+  DCHECK(!IsActiveTree());
+
+  if (reflected_main_frame_state_) {
+    for (const auto& scroll_update : reflected_main_frame_state_->scrolls) {
+      // The reflected scroll updates should have been sent only for layers
+      // in the main frame, and the updates are applied after the main frame is
+      // committed to the pending tree, so the layer must be present in the
+      // pending tree.
+      DCHECK(LayerById(scroll_update.layer_id));
+
+      property_trees()->scroll_tree.AddUnappliedScrollOffsetDeltaToPendingTree(
+          scroll_update.layer_id,
+          gfx::ScrollOffset(scroll_update.scroll_delta));
+      DidUpdateScrollOffset(scroll_update.layer_id);
+    }
+
+    float page_scale_delta = reflected_main_frame_state_->page_scale_delta;
+    if (page_scale_delta != 1.0f) {
+      page_scale_factor_->AddUnappliedDeltaToPendingBase(page_scale_delta);
+      DidUpdatePageScale();
+    }
+
+    reflected_main_frame_state_ = nullptr;
+  }
+}
+
 }  // namespace cc