Fix scroll chaining for non-descendants of root scroller.

cc/trees/layer_tree_host_in_process.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_in_process.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 432 matching lines @@
     sync_tree->ForceRedrawNextActivation();
     next_commit_forces_redraw_ = false;
   }
   if (next_commit_forces_recalculate_raster_scales_) {
     sync_tree->ForceRecalculateRasterScales();
     next_commit_forces_recalculate_raster_scales_ = false;
   }
 
   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_);
   host_impl->SetContentIsSuitableForGpuRasterization(
       content_is_suitable_for_gpu_rasterization_);
   RecordGpuRasterizationHistogram();
 
   host_impl->SetViewportSize(layer_tree_->device_viewport_size());
   // TODO(senorblanco): Move this to LayerTree::PushPropertiesTo so that it
   // happens before GPU rasterization properties are set, since those trigger an
@@ skipping 567 matching lines @@
 
   debug_state_.FromProtobuf(proto.debug_state());
   has_gpu_rasterization_trigger_ = proto.has_gpu_rasterization_trigger();
   content_is_suitable_for_gpu_rasterization_ =
       proto.content_is_suitable_for_gpu_rasterization();
   id_ = proto.id();
   next_commit_forces_redraw_ = proto.next_commit_forces_redraw();
 }
 
 }  // namespace cc