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

cc/trees/layer_tree_host_unittest_animation.cc

Index: cc/trees/layer_tree_host_unittest_animation.cc
diff --git a/cc/trees/layer_tree_host_unittest_animation.cc b/cc/trees/layer_tree_host_unittest_animation.cc
index 9547562a00eff715f0487896afeaa37be990da0f..11039c1da0315d9562febb04a3437cdd576d73a9 100644
--- a/cc/trees/layer_tree_host_unittest_animation.cc
+++ b/cc/trees/layer_tree_host_unittest_animation.cc
@@ -825,7 +825,7 @@ class LayerTreeHostAnimationTestScrollOffsetAnimationRemoval
         timeline_impl->GetPlayerById(player_child_id_);
 
     LayerImpl* scroll_layer_impl =
-        host_impl->active_tree()->root_layer()->children()[0];
+        host_impl->active_tree()->LayerById(scroll_layer_->id());
     Animation* animation = player_impl->element_animations()
                                ->GetAnimation(TargetProperty::SCROLL_OFFSET);
 
@@ -847,7 +847,7 @@ class LayerTreeHostAnimationTestScrollOffsetAnimationRemoval
     if (host_impl->pending_tree()->source_frame_number() != 1)
       return;
     LayerImpl* scroll_layer_impl =
-        host_impl->pending_tree()->root_layer()->children()[0];
+        host_impl->pending_tree()->LayerById(scroll_layer_->id());
     EXPECT_EQ(final_postion_, scroll_layer_impl->CurrentScrollOffset());
   }
 
@@ -855,7 +855,7 @@ class LayerTreeHostAnimationTestScrollOffsetAnimationRemoval
     if (host_impl->active_tree()->source_frame_number() != 1)
       return;
     LayerImpl* scroll_layer_impl =
-        host_impl->active_tree()->root_layer()->children()[0];
+        host_impl->active_tree()->LayerById(scroll_layer_->id());
     EXPECT_EQ(final_postion_, scroll_layer_impl->CurrentScrollOffset());
     EndTest();
   }
@@ -1015,8 +1015,7 @@ class LayerTreeHostAnimationTestPendingTreeAnimatesFirstCommit
     scoped_refptr<AnimationPlayer> player_impl =
         timeline_impl->GetPlayerById(player_id_);
 
-    LayerImpl* root = host_impl->sync_tree()->root_layer();
-    LayerImpl* child = root->children()[0];
+    LayerImpl* child = host_impl->sync_tree()->LayerById(layer_->id());
     Animation* animation = player_impl->element_animations()->GetAnimation(
         TargetProperty::TRANSFORM);
 
@@ -1173,7 +1172,7 @@ class LayerTreeHostAnimationTestAddAnimationAfterAnimating
       if (id == host_impl->RootLayer()->id()) {
         Animation* anim = it.second->GetAnimation(TargetProperty::TRANSFORM);
         EXPECT_GT((anim->start_time() - base::TimeTicks()).InSecondsF(), 0);
-      } else if (id == host_impl->RootLayer()->children()[0]->id()) {
+      } else if (id == layer_->id()) {
         Animation* anim = it.second->GetAnimation(TargetProperty::OPACITY);
         EXPECT_GT((anim->start_time() - base::TimeTicks()).InSecondsF(), 0);
       }
@@ -1237,8 +1236,7 @@ class LayerTreeHostAnimationTestRemoveAnimation
   }
 
   void DrawLayersOnThread(LayerTreeHostImpl* host_impl) override {
-    LayerImpl* root = host_impl->active_tree()->root_layer();
-    LayerImpl* child = root->children()[0];
+    LayerImpl* child = host_impl->active_tree()->LayerById(layer_->id());
     switch (host_impl->active_tree()->source_frame_number()) {
       case 0:
         // No animation yet.
@@ -1301,8 +1299,7 @@ class LayerTreeHostAnimationTestIsAnimating
   }
 
   void CommitCompleteOnThread(LayerTreeHostImpl* host_impl) override {
-    LayerImpl* root = host_impl->sync_tree()->root_layer();
-    LayerImpl* child = root->children()[0];
+    LayerImpl* child = host_impl->sync_tree()->LayerById(layer_->id());
     switch (host_impl->sync_tree()->source_frame_number()) {
       case 0:
         // No animation yet.
@@ -1322,8 +1319,7 @@ class LayerTreeHostAnimationTestIsAnimating
   }
 
   void DrawLayersOnThread(LayerTreeHostImpl* host_impl) override {
-    LayerImpl* root = host_impl->active_tree()->root_layer();
-    LayerImpl* child = root->children()[0];
+    LayerImpl* child = host_impl->active_tree()->LayerById(layer_->id());
     switch (host_impl->active_tree()->source_frame_number()) {
       case 0:
         // No animation yet.
@@ -1396,8 +1392,7 @@ class LayerTreeHostAnimationTestAnimationFinishesDuringCommit
       case 2:
         gfx::Transform expected_transform;
         expected_transform.Translate(5.f, 5.f);
-        LayerImpl* layer_impl =
-            host_impl->sync_tree()->root_layer()->children()[0];
+        LayerImpl* layer_impl = host_impl->sync_tree()->LayerById(layer_->id());
         EXPECT_EQ(expected_transform, layer_impl->DrawTransform());
         EndTest();
         break;