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

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 6c2f6a8aaaeb3734a52ffb8a15342223adfc3760..b8a271d9286edcb8cbfbd178de2411dd4f355774 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -208,6 +208,10 @@ void LayerTreeHostCommon::ScrollbarsUpdateInfo::FromProtobuf(
   hidden = proto.hidden();
 }
 
+ReflectedMainFrameState::ReflectedMainFrameState() : page_scale_delta(1.0f) {}
+
+ReflectedMainFrameState::~ReflectedMainFrameState() = default;
+
 ScrollAndScaleSet::ScrollAndScaleSet()
     : page_scale_delta(1.f), top_controls_delta(0.f) {
 }