cc : Calculate jitter without using layer hierarchy

cc/trees/layer_tree_host_impl_unittest.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_impl.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <cmath>
@@ skipping 10331 matching lines @@
     host_impl_->CreatePendingTree();
     host_impl_->pending_tree()->set_source_frame_number(i + 1);
     LayerImpl* content_layer =
         host_impl_->pending_tree()->OuterViewportScrollLayer()->children()[0];
     // The scroll done on the active tree is undone on the pending tree.
     gfx::Transform translate;
     translate.Translate(0, accumulated_scroll);
     content_layer->SetTransform(translate);
 
     LayerTreeImpl* pending_tree = host_impl_->pending_tree();
+    LayerTreeImpl* active_tree = host_impl_->active_tree();
     pending_tree->PushPageScaleFromMainThread(1.f, 1.f, 1.f);
-    LayerImpl* last_scrolled_layer = pending_tree->LayerById(
-        host_impl_->active_tree()->LastScrolledLayerId());
+    ScrollNode* node = active_tree->property_trees()->scroll_tree.Node(
+        active_tree->LastScrolledScrollNodeId());
+    LayerImpl* last_scrolled_layer = pending_tree->LayerById(node->owner_id);
 
     // When building property trees from impl side, the builder uses the scroll
     // offset of layer_impl to initialize the scroll offset in scroll tree:
     //  scroll_tree.synced_scroll_offset.PushFromMainThread(
     //                                   layer->CurrentScrollOffset()).
     // However, layer_impl does not store scroll_offset, so it is using scroll
     // tree's scroll offset to initialize itself. Usually this approach works
     // because this is a simple assignment. However if scroll_offset's pending
     // delta is not zero, the delta would be counted twice.
     // This hacking here is to restore the damaged scroll offset.
     gfx::ScrollOffset pending_base =
         pending_tree->property_trees()
             ->scroll_tree.GetScrollOffsetBaseForTesting(
                 last_scrolled_layer->id());
     pending_tree->property_trees()->needs_rebuild = true;
     pending_tree->BuildPropertyTreesForTesting();
     pending_tree->property_trees()
         ->scroll_tree.UpdateScrollOffsetBaseForTesting(
             last_scrolled_layer->id(), pending_base);
 
     pending_tree->set_needs_update_draw_properties();
     pending_tree->UpdateDrawProperties(false);
-    float jitter =
-        LayerTreeHostCommon::CalculateFrameJitter(last_scrolled_layer);
+    float jitter = LayerTreeHostCommon::CalculateFrameJitter(content_layer);
     // There should not be any jitter measured till we hit the fixed point hits
     // threshold.
     float expected_jitter =
         (i == pending_tree->kFixedPointHitsThreshold) ? 500 : 0;
     EXPECT_EQ(jitter, expected_jitter);
   }
 }
 
 }  // namespace
 }  // namespace cc