cc: Stop creating unused 0 property tree nodes other than transform

cc/trees/layer_tree_host_common_unittest.cc

Index: cc/trees/layer_tree_host_common_unittest.cc
diff --git a/cc/trees/layer_tree_host_common_unittest.cc b/cc/trees/layer_tree_host_common_unittest.cc
index 52386ed2559344eb92bbd3cdbaf2e1723b30a474..d654072d4dcbbd8c9bf9771af257637351f3234a 100644
--- a/cc/trees/layer_tree_host_common_unittest.cc
+++ b/cc/trees/layer_tree_host_common_unittest.cc
@@ -9837,7 +9837,7 @@ TEST_F(LayerTreeHostCommonTest, TwoUnclippedRenderSurfaces) {
   EXPECT_EQ(gfx::Rect(-10, -10, 30, 30), render_surface2->clip_rect());
   // A clip node is created for every render surface and for layers that have
   // local clip. So, here it should be craeted for every layer.
-  EXPECT_EQ(root->layer_tree_impl()->property_trees()->clip_tree.size(), 5u);
+  EXPECT_EQ(root->layer_tree_impl()->property_trees()->clip_tree.size(), 4u);
 }
 
 TEST_F(LayerTreeHostCommonTest, MaskLayerDrawProperties) {
@@ -10264,22 +10264,21 @@ TEST_F(LayerTreeHostCommonTest, ScrollTreeBuilderTest) {
   // +----parent5(5)[contains_non_fast_scrollable_region]
   //
   // Expected scroll tree topology:
-  // +property_tree_root---owner:-1
-  // +--root---owner:1, id:1
-  // +----node---owner:2, id:2
-  // +------node---owner:6, id:3
-  // +----node---owner:7, id:4
-  // +------node---owner:11, id:5
-  // +----node---owner:5, id:6
+  // +root---owner:1, id:0
+  // +--node---owner:2, id:1
+  // +----node---owner:6, id:2
+  // +--node---owner:7, id:3
+  // +----node---owner:11, id:4
+  // +--node---owner:5, id:5
   //
   // Extra check:
   //   scroll_tree_index() of:
-  //     grand_child10:3
-  //     parent3:1
-  //     child8:4
-  //     parent4:1
-  //     child9:1
-  //     grand_child12:1
+  //     grand_child10:2
+  //     parent3:0
+  //     child8:3
+  //     parent4:0
+  //     child9:0
+  //     grand_child12:0
   scoped_refptr<Layer> root1 = Layer::Create();
   scoped_refptr<Layer> page_scale_layer = Layer::Create();
   scoped_refptr<Layer> parent2 = Layer::Create();
@@ -10329,37 +10328,25 @@ TEST_F(LayerTreeHostCommonTest, ScrollTreeBuilderTest) {
   host()->RegisterViewportLayers(NULL, page_scale_layer, parent2, NULL);
   ExecuteCalculateDrawPropertiesWithPropertyTrees(root1.get());
 
-  const int kInvalidPropertyTreeNodeId = -1;
-  const int kRootPropertyTreeNodeId = 0;
-
   // Property tree root
   ScrollTree& scroll_tree = host()->property_trees()->scroll_tree;
   PropertyTrees property_trees;
   property_trees.is_main_thread = true;
   property_trees.is_active = false;
   ScrollTree& expected_scroll_tree = property_trees.scroll_tree;
-  ScrollNode* property_tree_root = expected_scroll_tree.Node(0);
-  property_tree_root->id = kRootPropertyTreeNodeId;
-  property_tree_root->parent_id = kInvalidPropertyTreeNodeId;
-  property_tree_root->owner_id = kInvalidPropertyTreeNodeId;
-  property_tree_root->scrollable = false;
-  property_tree_root->main_thread_scrolling_reasons =
-      MainThreadScrollingReason::kNotScrollingOnMain;
-  property_tree_root->contains_non_fast_scrollable_region = false;
-  property_tree_root->transform_id = kRootPropertyTreeNodeId;
 
   // The node owned by root1
   ScrollNode scroll_root1;
-  scroll_root1.id = 1;
+  scroll_root1.id = 0;
   scroll_root1.owner_id = root1->id();
   scroll_root1.user_scrollable_horizontal = true;
   scroll_root1.user_scrollable_vertical = true;
   scroll_root1.transform_id = root1->transform_tree_index();
-  expected_scroll_tree.Insert(scroll_root1, 0);
+  expected_scroll_tree.Insert(scroll_root1, -1);
 
   // The node owned by parent2
   ScrollNode scroll_parent2;
-  scroll_parent2.id = 2;
+  scroll_parent2.id = 1;
   scroll_parent2.owner_id = parent2->id();
   scroll_parent2.scrollable = true;
   scroll_parent2.main_thread_scrolling_reasons =
@@ -10371,11 +10358,11 @@ TEST_F(LayerTreeHostCommonTest, ScrollTreeBuilderTest) {
   scroll_parent2.user_scrollable_horizontal = true;
   scroll_parent2.user_scrollable_vertical = true;
   scroll_parent2.transform_id = parent2->transform_tree_index();
-  expected_scroll_tree.Insert(scroll_parent2, 1);
+  expected_scroll_tree.Insert(scroll_parent2, 0);
 
   // The node owned by child6
   ScrollNode scroll_child6;
-  scroll_child6.id = 3;
+  scroll_child6.id = 2;
   scroll_child6.owner_id = child6->id();
   scroll_child6.main_thread_scrolling_reasons =
       child6->main_thread_scrolling_reasons();
@@ -10383,11 +10370,11 @@ TEST_F(LayerTreeHostCommonTest, ScrollTreeBuilderTest) {
   scroll_child6.user_scrollable_horizontal = true;
   scroll_child6.user_scrollable_vertical = true;
   scroll_child6.transform_id = child6->transform_tree_index();
-  expected_scroll_tree.Insert(scroll_child6, 2);
+  expected_scroll_tree.Insert(scroll_child6, 1);
 
   // The node owned by child7, child7 also owns a transform node
   ScrollNode scroll_child7;
-  scroll_child7.id = 4;
+  scroll_child7.id = 3;
   scroll_child7.owner_id = child7->id();
   scroll_child7.scrollable = true;
   scroll_child7.scroll_clip_layer_bounds = parent3->bounds();
@@ -10395,21 +10382,21 @@ TEST_F(LayerTreeHostCommonTest, ScrollTreeBuilderTest) {
   scroll_child7.user_scrollable_horizontal = true;
   scroll_child7.user_scrollable_vertical = true;
   scroll_child7.transform_id = child7->transform_tree_index();
-  expected_scroll_tree.Insert(scroll_child7, 1);
+  expected_scroll_tree.Insert(scroll_child7, 0);
 
   // The node owned by grand_child11, grand_child11 also owns a transform node
   ScrollNode scroll_grand_child11;
-  scroll_grand_child11.id = 5;
+  scroll_grand_child11.id = 4;
   scroll_grand_child11.owner_id = grand_child11->id();
   scroll_grand_child11.scrollable = true;
   scroll_grand_child11.user_scrollable_horizontal = true;
   scroll_grand_child11.user_scrollable_vertical = true;
   scroll_grand_child11.transform_id = grand_child11->transform_tree_index();
-  expected_scroll_tree.Insert(scroll_grand_child11, 4);
+  expected_scroll_tree.Insert(scroll_grand_child11, 3);
 
   // The node owned by parent5
   ScrollNode scroll_parent5;
-  scroll_parent5.id = 8;
+  scroll_parent5.id = 5;
   scroll_parent5.owner_id = parent5->id();
   scroll_parent5.contains_non_fast_scrollable_region = true;
   scroll_parent5.bounds = gfx::Size(10, 10);
@@ -10417,7 +10404,7 @@ TEST_F(LayerTreeHostCommonTest, ScrollTreeBuilderTest) {
   scroll_parent5.user_scrollable_horizontal = true;
   scroll_parent5.user_scrollable_vertical = true;
   scroll_parent5.transform_id = parent5->transform_tree_index();
-  expected_scroll_tree.Insert(scroll_parent5, 1);
+  expected_scroll_tree.Insert(scroll_parent5, 0);
 
   expected_scroll_tree.SetScrollOffset(parent2->id(), gfx::ScrollOffset(0, 0));
   expected_scroll_tree.SetScrollOffset(child7->id(), gfx::ScrollOffset(0, 0));