Don't set sent_top_controls_delta when root layer doesn't exist.

cc/trees/layer_tree_host_common.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_common.h"
 
 #include <algorithm>
 
 #include "base/debug/trace_event.h"
 #include "cc/base/math_util.h"
 #include "cc/layers/heads_up_display_layer_impl.h"
 #include "cc/layers/layer.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/layer_iterator.h"
 #include "cc/layers/render_surface.h"
 #include "cc/layers/render_surface_impl.h"
 #include "cc/trees/layer_sorter.h"
 #include "cc/trees/layer_tree_impl.h"
 #include "ui/gfx/rect_conversions.h"
 #include "ui/gfx/transform.h"
 
 namespace cc {
 
-ScrollAndScaleSet::ScrollAndScaleSet() {}
+ScrollAndScaleSet::ScrollAndScaleSet()
+    : page_scale_delta(1.f), top_controls_delta(0.f) {
+}
 
 ScrollAndScaleSet::~ScrollAndScaleSet() {}
 
 static void SortLayers(LayerList::iterator forst,
                        LayerList::iterator end,
                        void* layer_sorter) {
   NOTREACHED();
 }
 
 static void SortLayers(LayerImplList::iterator first,
@@ skipping 2402 matching lines @@
       inputs->current_render_surface_layer_list_id);
 
   // The dummy layer list should not have been used.
   DCHECK_EQ(0u, dummy_layer_list.size());
   // A root layer render_surface should always exist after
   // CalculateDrawProperties.
   DCHECK(inputs->root_layer->render_surface());
 }
 
 }  // namespace cc