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..2c26be1c656677899ee029e5cb9c8b83a31e01fe 100644
--- a/cc/trees/layer_tree_host_unittest_picture.cc
+++ b/cc/trees/layer_tree_host_unittest_picture.cc
@@ -36,6 +36,7 @@ class LayerTreeHostPictureTestTwinLayer
     : public LayerTreeHostPictureTest {
   void SetupTree() override {
     SetupTreeWithSinglePictureLayer(gfx::Size(1, 1));
+    picture_id = root_picture_layer_->id();
   }
 
   void BeginTest() override {
@@ -52,13 +53,14 @@ 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_id = picture->id();

[ajuma, 2016/04/20 21:40:52]

Changing picture_id on the main thread will accessing it on the compositor
thread is something to avoid (even if there's an argument that it happens to be
safe in this case). Please use two different variables, and use
source_frame_number on the compositor side to decide which one to use.

[sunxd, 2016/04/21 17:31:16]

Done.

         layer_tree_host()->root_layer()->AddChild(picture);
         break;
       }
@@ -73,16 +75,16 @@ 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();
 
-    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 +92,15 @@ 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 +114,12 @@ class LayerTreeHostPictureTestTwinLayer
   }
 
   void DidActivateTreeOnThread(LayerTreeHostImpl* impl) override {
-    LayerImpl* active_root_impl = impl->active_tree()->root_layer();
-    if (active_root_impl->children().empty()) {
+    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 +129,7 @@ class LayerTreeHostPictureTestTwinLayer
   void AfterTest() override { EXPECT_EQ(5, activates_); }
 
   int activates_;
+  int picture_id;

[ajuma, 2016/04/20 21:40:52]

nit: picture_id_

[sunxd, 2016/04/21 17:31:16]

Done.

 };
 
 // There is no pending layers in single thread mode.
@@ -153,7 +157,7 @@ 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 +226,7 @@ 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 +275,7 @@ 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 +329,7 @@ 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 +349,7 @@ 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 +434,7 @@ 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 +459,7 @@ 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_)