Find root scroll layer at tree activation

cc/layer_tree_host.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/layer_tree_host.h"
 
 #include "base/command_line.h"
 #include "base/debug/trace_event.h"
 #include "base/message_loop.h"
 #include "base/stl_util.h"
@@ skipping 267 matching lines @@
         syncTree = hostImpl->activeTree();
         needsFullTreeSync = m_needsFullTreeSync;
     }
 
     if (needsFullTreeSync)
         syncTree->SetRootLayer(TreeSynchronizer::synchronizeTrees(rootLayer(), syncTree->DetachLayerTree(), syncTree));
     {
         TRACE_EVENT0("cc", "LayerTreeHost::pushProperties");
         TreeSynchronizer::pushProperties(rootLayer(), syncTree->RootLayer());
     }
-    syncTree->FindRootScrollLayer();
 
     m_needsFullTreeSync = false;
 
     if (m_rootLayer && m_hudLayer)
         syncTree->set_hud_layer(static_cast<HeadsUpDisplayLayerImpl*>(LayerTreeHostCommon::findLayerInSubtree(syncTree->RootLayer(), m_hudLayer->id())));
     else
         syncTree->set_hud_layer(0);
 
     syncTree->set_source_frame_number(commitNumber());
     syncTree->set_background_color(m_backgroundColor);
     syncTree->set_has_transparent_background(m_hasTransparentBackground);
 
+    syncTree->FindRootScrollLayer();
+
     if (!m_settings.implSidePainting) {
         // If we're not in impl-side painting, the tree is immediately
         // considered active.
         syncTree->DidBecomeActive();
     }
 
     hostImpl->setViewportSize(layoutViewportSize(), deviceViewportSize());
     hostImpl->setDeviceScaleFactor(deviceScaleFactor());
     hostImpl->setPageScaleFactorAndLimits(m_pageScaleFactor, m_minPageScaleFactor, m_maxPageScaleFactor);
     hostImpl->setDebugState(m_debugState);
@@ skipping 565 matching lines @@
     for (size_t childIndex = 0; childIndex < layer->children().size(); ++childIndex)
         setAnimationEventsRecursive(events, layer->children()[childIndex].get(), wallClockTime);
 }
 
 skia::RefPtr<SkPicture> LayerTreeHost::capturePicture()
 {
     return m_proxy->capturePicture();
 }
 
 }  // namespace cc