Fix scroll chaining for non-descendants of root scroller.

cc/trees/layer_tree_host_in_process.cc

Index: cc/trees/layer_tree_host_in_process.cc
diff --git a/cc/trees/layer_tree_host_in_process.cc b/cc/trees/layer_tree_host_in_process.cc
index f980b489a44074c12908f8d2316ed967c7d96084..f420f887c1a88ad88704621d95c7c6f85e451b05 100644
--- a/cc/trees/layer_tree_host_in_process.cc
+++ b/cc/trees/layer_tree_host_in_process.cc
@@ -450,11 +450,24 @@ void LayerTreeHostInProcess::FinishCommitOnImplThread(
 
   sync_tree->set_source_frame_number(SourceFrameNumber());
 
+  // TODO(bokan): The tree sync might destroy the currently scrolling layer.
+  // Normally, this would be fine and we'd just drop incoming scrolls but some
+  // telemetry tests rely on the viewport not having this behavior. Those should
+  // be fixed and this removed. crbug.com/652905.
+  bool restore_currently_scrolling_viewport =
+      host_impl->IsCurrentlyScrollingViewport();
+
   if (layer_tree_->needs_full_tree_sync())
     TreeSynchronizer::SynchronizeTrees(layer_tree_->root_layer(), sync_tree);
 
   layer_tree_->PushPropertiesTo(sync_tree);
 
+  if (restore_currently_scrolling_viewport &&
+      !host_impl->IsCurrentlyScrollingViewport()) {
+    host_impl->active_tree()->SetCurrentlyScrollingLayer(
+        host_impl->ViewportMainScrollLayer());
+  }
+
   sync_tree->PassSwapPromises(swap_promise_manager_.TakeSwapPromises());
 
   host_impl->SetHasGpuRasterizationTrigger(has_gpu_rasterization_trigger_);