cc: Remove LayerImpl::children() calls after tree synchronization

cc/layers/layer_position_constraint_unittest.cc

Index: cc/layers/layer_position_constraint_unittest.cc
diff --git a/cc/layers/layer_position_constraint_unittest.cc b/cc/layers/layer_position_constraint_unittest.cc
index 9c060eab43a99ccfceb9131965b9e74cd60ab6a1..f61bd85e892387eb1f4914c3f30f673b29abee9e 100644
--- a/cc/layers/layer_position_constraint_unittest.cc
+++ b/cc/layers/layer_position_constraint_unittest.cc
@@ -155,14 +155,18 @@ class LayerPositionConstraintTest : public testing::Test {
       SetScrollOffsetDelta(grand_child_impl_, gfx::Vector2dF());
     }
     root_impl_ = layer_tree_host_->CommitAndCreateLayerImplTree();
-    inner_viewport_container_layer_impl_ = root_impl_->children()[0];
-    scroll_layer_impl_ = inner_viewport_container_layer_impl_->children()[0];
-    outer_viewport_container_layer_impl_ = scroll_layer_impl_->children()[0];
+    layer_tree_impl_ = root_impl_->layer_tree_impl();
+    inner_viewport_container_layer_impl_ =
+        layer_tree_impl_->LayerById(inner_viewport_container_layer_->id());
+    scroll_layer_impl_ = layer_tree_impl_->LayerById(scroll_layer_->id());
+    outer_viewport_container_layer_impl_ =
+        layer_tree_impl_->LayerById(outer_viewport_container_layer_->id());
     child_transform_layer_impl_ =
-        outer_viewport_container_layer_impl_->children()[0];
-    child_impl_ = child_transform_layer_impl_->children()[0];
-    grand_child_impl_ = child_impl_->children()[0];
-    great_grand_child_impl_ = grand_child_impl_->children()[0];
+        layer_tree_impl_->LayerById(child_transform_layer_->id());
+    child_impl_ = layer_tree_impl_->LayerById(child_->id());
+    grand_child_impl_ = layer_tree_impl_->LayerById(grand_child_->id());
+    great_grand_child_impl_ =
+        layer_tree_impl_->LayerById(great_grand_child_->id());
   }
 
  protected:
@@ -177,6 +181,7 @@ class LayerPositionConstraintTest : public testing::Test {
   scoped_refptr<Layer> child_;
   scoped_refptr<Layer> grand_child_;
   scoped_refptr<Layer> great_grand_child_;
+  LayerTreeImpl* layer_tree_impl_;
   LayerImpl* root_impl_;
   LayerImpl* inner_viewport_container_layer_impl_;
   LayerImpl* scroll_layer_impl_;
@@ -575,7 +580,8 @@ TEST_F(LayerPositionConstraintTest,
   fixed_position_child->SetTransform(rotation_about_z);
 
   CommitAndUpdateImplPointers();
-  LayerImpl* fixed_position_child_impl = great_grand_child_impl_->children()[0];
+  LayerImpl* fixed_position_child_impl =
+      layer_tree_impl_->LayerById(fixed_position_child->id());
 
   // Case 1: scroll delta of 0, 0
   SetScrollOffsetDelta(child_impl_, gfx::Vector2d(0, 0));
@@ -671,7 +677,8 @@ TEST_F(LayerPositionConstraintTest,
   // Case 4: Bottom-right fixed-position layer.
   fixed_position_child->SetPositionConstraint(fixed_to_bottom_right_);
   CommitAndUpdateImplPointers();
-  fixed_position_child_impl = great_grand_child_impl_->children()[0];
+  fixed_position_child_impl =
+      layer_tree_impl_->LayerById(fixed_position_child->id());
   SetScrollOffsetDelta(child_impl_, gfx::Vector2d(10, 30));
   SetFixedContainerSizeDelta(child_impl_, gfx::Vector2d(20, 20));
   ExecuteCalculateDrawProperties(root_impl_);
@@ -732,7 +739,8 @@ TEST_F(
   fixed_position_child->SetTransform(rotation_about_z);
 
   CommitAndUpdateImplPointers();
-  LayerImpl* fixed_position_child_impl = great_grand_child_impl_->children()[0];
+  LayerImpl* fixed_position_child_impl =
+      layer_tree_impl_->LayerById(fixed_position_child->id());
 
   // Case 1: scroll delta of 0, 0
   SetScrollOffsetDelta(child_impl_, gfx::Vector2d(0, 0));
@@ -1051,7 +1059,8 @@ TEST_F(LayerPositionConstraintTest,
   LayerImpl* container1 = child_impl_;
   LayerImpl* fixed_to_container1 = grand_child_impl_;
   LayerImpl* container2 = great_grand_child_impl_;
-  LayerImpl* fixed_to_container2 = container2->children()[0];
+  LayerImpl* fixed_to_container2 =
+      layer_tree_impl_->LayerById(great_great_grand_child->id());
 
   SetScrollOffsetDelta(container1, gfx::Vector2d(0, 15));
   container1->SetDrawsContent(true);