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 3120dbfe34e8e6d4bf7f5d4c13a6bf9741ef68c6..0ed9cf4e4d2923b6dcdb9c73ac4089dbbdd83585 100644
--- a/cc/layers/scrollbar_layer_unittest.cc
+++ b/cc/layers/scrollbar_layer_unittest.cc
@@ -386,6 +386,7 @@ 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());
@@ -753,9 +754,7 @@ TEST_F(ScrollbarLayerTest, ScrollbarLayerPushProperties) {
   scroll_layer->SetBounds(gfx::Size(20, 20));
   scroll_layer->SetForceRenderSurfaceForTesting(true);
   layer_tree_host_->UpdateLayers();
-  host_impl->CreatePendingTree();
-  layer_tree_host_->CommitAndCreatePendingTree();
-  host_impl->ActivateSyncTree();
+  layer_tree_host_->CommitAndCreateLayerImplTree();
   EffectNode* node =
       host_impl->active_tree()->property_trees()->effect_tree.Node(
           scrollbar_layer->effect_tree_index());