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

cc/trees/layer_tree_host_unittest_damage.cc

Index: cc/trees/layer_tree_host_unittest_damage.cc
diff --git a/cc/trees/layer_tree_host_unittest_damage.cc b/cc/trees/layer_tree_host_unittest_damage.cc
index 76c26f6682e9c35bf8fdbf75506167a702213575..a9afb92b67c93c98150989cef22a5b07c71f7824 100644
--- a/cc/trees/layer_tree_host_unittest_damage.cc
+++ b/cc/trees/layer_tree_host_unittest_damage.cc
@@ -338,26 +338,26 @@ class LayerTreeHostScrollbarDamageTest : public LayerTreeHostDamageTest {
     layer_tree_host()->SetRootLayer(root_layer);
 
     scoped_refptr<Layer> scroll_clip_layer = Layer::Create();
-    scoped_refptr<Layer> content_layer = FakePictureLayer::Create(&client_);
-    content_layer->SetScrollClipLayerId(scroll_clip_layer->id());
-    content_layer->SetScrollOffset(gfx::ScrollOffset(10, 20));
-    content_layer->SetBounds(gfx::Size(100, 200));
-    content_layer->SetIsDrawable(true);
+    content_layer_ = FakePictureLayer::Create(&client_);
+    content_layer_->SetScrollClipLayerId(scroll_clip_layer->id());
+    content_layer_->SetScrollOffset(gfx::ScrollOffset(10, 20));
+    content_layer_->SetBounds(gfx::Size(100, 200));
+    content_layer_->SetIsDrawable(true);
     scroll_clip_layer->SetBounds(
-        gfx::Size(content_layer->bounds().width() - 30,
-                  content_layer->bounds().height() - 50));
-    scroll_clip_layer->AddChild(content_layer);
+        gfx::Size(content_layer_->bounds().width() - 30,
+                  content_layer_->bounds().height() - 50));
+    scroll_clip_layer->AddChild(content_layer_);
     root_layer->AddChild(scroll_clip_layer);
 
     scoped_refptr<Layer> scrollbar_layer =
-        FakePaintedScrollbarLayer::Create(false, true, content_layer->id());
+        FakePaintedScrollbarLayer::Create(false, true, content_layer_->id());
     scrollbar_layer->SetPosition(gfx::PointF(300.f, 300.f));
     scrollbar_layer->SetBounds(gfx::Size(10, 100));
-    scrollbar_layer->ToScrollbarLayer()->SetScrollLayer(content_layer->id());
+    scrollbar_layer->ToScrollbarLayer()->SetScrollLayer(content_layer_->id());
     root_layer->AddChild(scrollbar_layer);
 
-    gfx::RectF content_rect(content_layer->position(),
-                            gfx::SizeF(content_layer->bounds()));
+    gfx::RectF content_rect(content_layer_->position(),
+                            gfx::SizeF(content_layer_->bounds()));
     gfx::RectF scrollbar_rect(scrollbar_layer->position(),
                               gfx::SizeF(scrollbar_layer->bounds()));
     EXPECT_FALSE(content_rect.Intersects(scrollbar_rect));
@@ -368,6 +368,9 @@ class LayerTreeHostScrollbarDamageTest : public LayerTreeHostDamageTest {
 
  private:
   FakeContentLayerClient client_;
+
+ protected:
+  scoped_refptr<Layer> content_layer_;
 };
 
 class LayerTreeHostDamageTestScrollbarDoesDamage
@@ -410,9 +413,8 @@ class LayerTreeHostDamageTestScrollbarDoesDamage
   void SwapBuffersOnThread(LayerTreeHostImpl* host_impl, bool result) override {
     ++did_swaps_;
     EXPECT_TRUE(result);
-    LayerImpl* root = host_impl->active_tree()->root_layer();
-    LayerImpl* scroll_clip_layer = root->children()[0];
-    LayerImpl* scroll_layer = scroll_clip_layer->children()[0];
+    LayerImpl* scroll_layer =
+        host_impl->active_tree()->LayerById(content_layer_->id());
     switch (did_swaps_) {
       case 1:
         // Test that modifying the position of the content layer (not
@@ -439,18 +441,13 @@ class LayerTreeHostDamageTestScrollbarDoesDamage
 
   void ModifyContentLayerPosition() {
     EXPECT_EQ(1, did_swaps_);
-    Layer* root = layer_tree_host()->root_layer();
-    Layer* scroll_clip_layer = root->child_at(0);
-    Layer* scroll_layer = scroll_clip_layer->child_at(0);
-    scroll_layer->SetPosition(gfx::PointF(10.f, 10.f));
+    content_layer_->SetPosition(gfx::PointF(10.f, 10.f));
   }
 
   void ResizeScrollLayer() {
     EXPECT_EQ(3, did_swaps_);
     Layer* root = layer_tree_host()->root_layer();
-    Layer* scroll_clip_layer = root->child_at(0);
-    Layer* scroll_layer = scroll_clip_layer->child_at(0);
-    scroll_layer->SetBounds(
+    content_layer_->SetBounds(
         gfx::Size(root->bounds().width() + 60, root->bounds().height() + 100));
   }
 
@@ -503,9 +500,8 @@ class LayerTreeHostDamageTestScrollbarCommitDoesNoDamage
   void SwapBuffersOnThread(LayerTreeHostImpl* host_impl, bool result) override {
     ++did_swaps_;
     EXPECT_TRUE(result);
-    LayerImpl* root = host_impl->active_tree()->root_layer();
-    LayerImpl* scroll_clip_layer = root->children()[0];
-    LayerImpl* scroll_layer = scroll_clip_layer->children()[0];
+    LayerImpl* scroll_layer =
+        host_impl->active_tree()->LayerById(content_layer_->id());
     switch (did_swaps_) {
       case 1:
         // Scroll on the thread.  This should damage the scrollbar for the