cc : Add DCHECKs to ensure we have valid proprety tree indices at commit

cc/layers/scrollbar_layer_unittest.cc

Index: cc/layers/scrollbar_layer_unittest.cc
diff --git a/cc/layers/scrollbar_layer_unittest.cc b/cc/layers/scrollbar_layer_unittest.cc
index da575b259c0fbeb5855b84e753ca01450cdde05d..6e8cf7046ab06fec38c48a317c0c89b4c13b9647 100644
--- a/cc/layers/scrollbar_layer_unittest.cc
+++ b/cc/layers/scrollbar_layer_unittest.cc
@@ -385,6 +385,7 @@ TEST_F(ScrollbarLayerTest, UpdatePropertiesOfScrollBarWhenThumbRemoved) {
   LayerImpl* root_clip_layer_impl = nullptr;
   PaintedScrollbarLayerImpl* scrollbar_layer_impl = nullptr;
 
+  layer_tree_host_->BuildPropertyTreesForTesting();
   UPDATE_AND_EXTRACT_LAYER_POINTERS();
   EXPECT_EQ(gfx::Rect(10, 0, 4, 10).ToString(),
             scrollbar_layer_impl->ComputeThumbQuadRect().ToString());
@@ -744,8 +745,10 @@ TEST_F(ScrollbarLayerTest, ScrollbarLayerPushProperties) {
   layer_tree_root->SetBounds(gfx::Size(2, 2));
   scroll_layer->SetBounds(gfx::Size(10, 10));
   layer_tree_host_->UpdateLayers();
-  layer_tree_host_->CommitAndCreateLayerImplTree();
   LayerTreeHostImpl* host_impl = layer_tree_host_->host_impl();
+  host_impl->CreatePendingTree();
+  layer_tree_host_->CommitAndCreatePendingTree();
+  host_impl->ActivateSyncTree();
   EXPECT_TRUE(host_impl->ScrollbarAnimationControllerForElementId(
       scroll_layer->element_id()));