cc: Remove LayerImpl::children() calls from descendants of LayerTreeTest

cc/trees/layer_tree_host_unittest_context.cc

Index: cc/trees/layer_tree_host_unittest_context.cc
diff --git a/cc/trees/layer_tree_host_unittest_context.cc b/cc/trees/layer_tree_host_unittest_context.cc
index ae08b6d111851f2df4107eb9c4fbc8026a9f5fb4..15ef05e0d6b3b41a35062a8e749b44d6fe777a8f 100644
--- a/cc/trees/layer_tree_host_unittest_context.cc
+++ b/cc/trees/layer_tree_host_unittest_context.cc
@@ -646,7 +646,7 @@ class LayerTreeHostContextTestLostContextSucceedsWithContent
 
   void DrawLayersOnThread(LayerTreeHostImpl* host_impl) override {
     FakePictureLayerImpl* picture_impl = static_cast<FakePictureLayerImpl*>(
-        host_impl->active_tree()->root_layer()->children()[0]);
+        host_impl->active_tree()->LayerById(layer_->id()));
     EXPECT_TRUE(picture_impl->HighResTiling()
                     ->TileAt(0, 0)
                     ->draw_info()
@@ -856,10 +856,10 @@ class LayerTreeHostContextTestLayersNotified : public LayerTreeHostContextTest {
 
     root_picture = static_cast<FakePictureLayerImpl*>(
         host_impl->active_tree()->root_layer());
-    child_picture =
-        static_cast<FakePictureLayerImpl*>(root_picture->children()[0]);
-    grandchild_picture =
-        static_cast<FakePictureLayerImpl*>(child_picture->children()[0]);
+    child_picture = static_cast<FakePictureLayerImpl*>(
+        host_impl->active_tree()->LayerById(child_->id()));
+    grandchild_picture = static_cast<FakePictureLayerImpl*>(
+        host_impl->active_tree()->LayerById(grandchild_->id()));
 
     ++num_commits_;
     switch (num_commits_) {