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

cc/trees/layer_tree_host_unittest_picture.cc

Index: cc/trees/layer_tree_host_unittest_picture.cc
diff --git a/cc/trees/layer_tree_host_unittest_picture.cc b/cc/trees/layer_tree_host_unittest_picture.cc
index c41a25259cd2bcf41472ca67d66cb6ad841afab5..319e609aef7478897865941acb6fbc749e6b89d1 100644
--- a/cc/trees/layer_tree_host_unittest_picture.cc
+++ b/cc/trees/layer_tree_host_unittest_picture.cc
@@ -36,6 +36,8 @@ class LayerTreeHostPictureTestTwinLayer
     : public LayerTreeHostPictureTest {
   void SetupTree() override {
     SetupTreeWithSinglePictureLayer(gfx::Size(1, 1));
+    picture_id1_ = root_picture_layer_->id();
+    picture_id2_ = -1;
   }
 
   void BeginTest() override {
@@ -52,13 +54,14 @@ class LayerTreeHostPictureTestTwinLayer
       case 2:
         // Drop the picture layer from the tree so the activate will have an
         // active layer without a pending twin.
-        layer_tree_host()->root_layer()->children()[0]->RemoveFromParent();
+        root_picture_layer_->RemoveFromParent();
         break;
       case 3: {
         // Add a new picture layer so the activate will have a pending layer
         // without an active twin.
         scoped_refptr<FakePictureLayer> picture =
             FakePictureLayer::Create(&client_);
+        picture_id2_ = picture->id();
         layer_tree_host()->root_layer()->AddChild(picture);
         break;
       }
@@ -73,16 +76,19 @@ class LayerTreeHostPictureTestTwinLayer
   }
 
   void WillActivateTreeOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* pending_root_impl = impl->pending_tree()->root_layer();
     LayerImpl* active_root_impl = impl->active_tree()->root_layer();
+    int picture_id = impl->active_tree()->source_frame_number() < 2
+                         ? picture_id1_
+                         : picture_id2_;
 
-    if (pending_root_impl->children().empty()) {
+    if (!impl->pending_tree()->LayerById(picture_id)) {
       EXPECT_EQ(2, activates_);
       return;
     }
 
     FakePictureLayerImpl* pending_picture_impl =
-        static_cast<FakePictureLayerImpl*>(pending_root_impl->children()[0]);
+        static_cast<FakePictureLayerImpl*>(
+            impl->pending_tree()->LayerById(picture_id));
 
     if (!active_root_impl) {
       EXPECT_EQ(0, activates_);
@@ -90,14 +96,15 @@ class LayerTreeHostPictureTestTwinLayer
       return;
     }
 
-    if (active_root_impl->children().empty()) {
+    if (!impl->active_tree()->LayerById(picture_id)) {
       EXPECT_EQ(3, activates_);
       EXPECT_EQ(nullptr, pending_picture_impl->GetPendingOrActiveTwinLayer());
       return;
     }
 
     FakePictureLayerImpl* active_picture_impl =
-        static_cast<FakePictureLayerImpl*>(active_root_impl->children()[0]);
+        static_cast<FakePictureLayerImpl*>(
+            impl->active_tree()->LayerById(picture_id));
 
     // After the first activation, when we commit again, we'll have a pending
     // and active layer. Then we recreate a picture layer in the 4th activate
@@ -111,12 +118,15 @@ class LayerTreeHostPictureTestTwinLayer
   }
 
   void DidActivateTreeOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* active_root_impl = impl->active_tree()->root_layer();
-    if (active_root_impl->children().empty()) {
+    int picture_id = impl->active_tree()->source_frame_number() < 3
+                         ? picture_id1_
+                         : picture_id2_;
+    if (!impl->active_tree()->LayerById(picture_id)) {
       EXPECT_EQ(2, activates_);
     } else {
       FakePictureLayerImpl* active_picture_impl =
-          static_cast<FakePictureLayerImpl*>(active_root_impl->children()[0]);
+          static_cast<FakePictureLayerImpl*>(
+              impl->active_tree()->LayerById(picture_id));
       EXPECT_EQ(nullptr, active_picture_impl->GetPendingOrActiveTwinLayer());
     }
 
@@ -126,6 +136,8 @@ class LayerTreeHostPictureTestTwinLayer
   void AfterTest() override { EXPECT_EQ(5, activates_); }
 
   int activates_;
+  int picture_id1_;
+  int picture_id2_;
 };
 
 // There is no pending layers in single thread mode.
@@ -153,7 +165,7 @@ class LayerTreeHostPictureTestResizeViewportWithGpuRaster
   void BeginTest() override { PostSetNeedsCommitToMainThread(); }
 
   void CommitCompleteOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* child = impl->sync_tree()->root_layer()->children()[0];
+    LayerImpl* child = impl->sync_tree()->LayerById(picture_->id());
     FakePictureLayerImpl* picture_impl =
         static_cast<FakePictureLayerImpl*>(child);
     gfx::Size tile_size =
@@ -222,7 +234,7 @@ class LayerTreeHostPictureTestChangeLiveTilesRectWithRecycleTree
   void BeginTest() override { PostSetNeedsCommitToMainThread(); }
 
   void DrawLayersOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* child = impl->active_tree()->root_layer()->children()[0];
+    LayerImpl* child = impl->active_tree()->LayerById(picture_->id());
     FakePictureLayerImpl* picture_impl =
         static_cast<FakePictureLayerImpl*>(child);
     switch (++frame_) {
@@ -271,7 +283,7 @@ class LayerTreeHostPictureTestChangeLiveTilesRectWithRecycleTree
   }
 
   void WillActivateTreeOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* child = impl->sync_tree()->root_layer()->children()[0];
+    LayerImpl* child = impl->sync_tree()->LayerById(picture_->id());
     FakePictureLayerImpl* picture_impl =
         static_cast<FakePictureLayerImpl*>(child);
     PictureLayerTiling* tiling = picture_impl->HighResTiling();
@@ -325,9 +337,7 @@ class LayerTreeHostPictureTestRSLLMembership : public LayerTreeHostPictureTest {
   void BeginTest() override { PostSetNeedsCommitToMainThread(); }
 
   void CommitCompleteOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* root = impl->sync_tree()->root_layer();
-    LayerImpl* child = root->children()[0];
-    LayerImpl* gchild = child->children()[0];
+    LayerImpl* gchild = impl->sync_tree()->LayerById(picture_->id());
     FakePictureLayerImpl* picture = static_cast<FakePictureLayerImpl*>(gchild);
 
     switch (impl->sync_tree()->source_frame_number()) {
@@ -347,9 +357,7 @@ class LayerTreeHostPictureTestRSLLMembership : public LayerTreeHostPictureTest {
   }
 
   void DidActivateTreeOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* root = impl->active_tree()->root_layer();
-    LayerImpl* child = root->children()[0];
-    LayerImpl* gchild = child->children()[0];
+    LayerImpl* gchild = impl->sync_tree()->LayerById(picture_->id());
     FakePictureLayerImpl* picture = static_cast<FakePictureLayerImpl*>(gchild);
 
     switch (impl->active_tree()->source_frame_number()) {
@@ -434,9 +442,7 @@ class LayerTreeHostPictureTestRSLLMembershipWithScale
   }
 
   void WillActivateTreeOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* root = impl->sync_tree()->root_layer();
-    LayerImpl* pinch = root->children()[0]->children()[0];
-    LayerImpl* gchild = pinch->children()[0];
+    LayerImpl* gchild = impl->sync_tree()->LayerById(picture_->id());
     FakePictureLayerImpl* picture = static_cast<FakePictureLayerImpl*>(gchild);
     ready_to_draw_ = false;
 
@@ -461,9 +467,7 @@ class LayerTreeHostPictureTestRSLLMembershipWithScale
   }
 
   void DrawLayersOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* root = impl->active_tree()->root_layer();
-    LayerImpl* pinch = root->children()[0]->children()[0];
-    LayerImpl* gchild = pinch->children()[0];
+    LayerImpl* gchild = impl->active_tree()->LayerById(picture_->id());
     FakePictureLayerImpl* picture = static_cast<FakePictureLayerImpl*>(gchild);
 
     if (frame_ != last_frame_drawn_)