cc: Remove LayerTreeImpl::root_layer usage outside tests

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index f558733c589de32d5ec3e280cf9f4788a7189439..f2cabde35d76da331d9b81b5f824803a2627cd1e 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -398,9 +398,9 @@ bool LayerTreeHostImpl::CanDraw() const {
   // Must have an OutputSurface if |renderer_| is not NULL.
   DCHECK(output_surface_);
 
-  // TODO(boliu): Make draws without root_layer work and move this below
+  // TODO(boliu): Make draws without layers work and move this below
   // |resourceless_software_draw_| check. Tracked in crbug.com/264967.
-  if (!active_tree_->root_layer()) {
+  if (active_tree_->LayerListIsEmpty()) {
     TRACE_EVENT_INSTANT0("cc", "LayerTreeHostImpl::CanDraw no root layer",
                          TRACE_EVENT_SCOPE_THREAD);
     return false;
@@ -632,7 +632,6 @@ void LayerTreeHostImpl::QueueSwapPromiseForMainThreadScrollUpdate(
 }
 
 void LayerTreeHostImpl::TrackDamageForAllSurfaces(
-    LayerImpl* root_draw_layer,
     const LayerImplList& render_surface_layer_list) {
   // For now, we use damage tracking to compute a global scissor. To do this, we
   // must compute all damage tracking before drawing anything, so that we know
@@ -730,10 +729,10 @@ static void AppendQuadsForRenderSurfaceLayer(
 
 static void AppendQuadsToFillScreen(const gfx::Rect& root_scroll_layer_rect,
                                     RenderPass* target_render_pass,
-                                    LayerImpl* root_layer,
+                                    RenderSurfaceImpl* root_render_surface,
                                     SkColor screen_background_color,
                                     const Region& fill_region) {
-  if (!root_layer || !SkColorGetA(screen_background_color))
+  if (!root_render_surface || !SkColorGetA(screen_background_color))
     return;
   if (fill_region.IsEmpty())
     return;
@@ -743,7 +742,7 @@ static void AppendQuadsToFillScreen(const gfx::Rect& root_scroll_layer_rect,
   // TODO(danakj): Make the gutter quads generated by the solid color layer
   // (make it smarter about generating quads to fill unoccluded areas).
 
-  gfx::Rect root_target_rect = root_layer->render_surface()->content_rect();
+  gfx::Rect root_target_rect = root_render_surface->content_rect();
   float opacity = 1.f;
   int sorting_context_id = 0;
   SharedQuadState* shared_quad_state =
@@ -785,15 +784,13 @@ DrawResult LayerTreeHostImpl::CalculateRenderPasses(
     FrameData* frame) {
   DCHECK(frame->render_passes.empty());
   DCHECK(CanDraw());
-  DCHECK(active_tree_->root_layer());
+  DCHECK(!active_tree_->LayerListIsEmpty());
 
-  TrackDamageForAllSurfaces(active_tree_->root_layer(),
-                            *frame->render_surface_layer_list);
+  TrackDamageForAllSurfaces(*frame->render_surface_layer_list);
 
   // If the root render surface has no visible damage, then don't generate a
   // frame at all.
-  RenderSurfaceImpl* root_surface =
-      active_tree_->root_layer()->render_surface();
+  RenderSurfaceImpl* root_surface = active_tree_->RootRenderSurface();
   bool root_surface_has_no_visible_damage =
       !root_surface->damage_tracker()->current_damage_rect().Intersects(
           root_surface->content_rect());
@@ -986,7 +983,7 @@ DrawResult LayerTreeHostImpl::CalculateRenderPasses(
     frame->render_passes.back()->has_transparent_background = false;
     AppendQuadsToFillScreen(
         active_tree_->RootScrollLayerDeviceViewportBounds(),
-        frame->render_passes.back().get(), active_tree_->root_layer(),
+        frame->render_passes.back().get(), active_tree_->RootRenderSurface(),
         active_tree_->background_color(), unoccluded_screen_space_region);
   }
 
@@ -1107,12 +1104,12 @@ DrawResult LayerTreeHostImpl::PrepareToDraw(FrameData* frame) {
   frame->will_draw_layers.clear();
   frame->has_no_damage = false;
 
-  if (active_tree_->root_layer()) {
+  if (active_tree_->RootRenderSurface()) {
     gfx::Rect device_viewport_damage_rect = viewport_damage_rect_;
     viewport_damage_rect_ = gfx::Rect();
 
-    active_tree_->root_layer()->render_surface()->damage_tracker()->
-        AddDamageNextUpdate(device_viewport_damage_rect);
+    active_tree_->RootRenderSurface()->damage_tracker()->AddDamageNextUpdate(
+        device_viewport_damage_rect);
   }
 
   DrawResult draw_result = CalculateRenderPasses(frame);
@@ -1646,10 +1643,8 @@ void LayerTreeHostImpl::DrawLayers(FrameData* frame) {
 
   if (debug_state_.ShowHudRects()) {
     debug_rect_history_->SaveDebugRectsForCurrentFrame(
-        active_tree_->root_layer(),
-        active_tree_->hud_layer(),
-        *frame->render_surface_layer_list,
-        debug_state_);
+        active_tree(), active_tree_->hud_layer(),
+        *frame->render_surface_layer_list, debug_state_);
   }
 
   bool is_new_trace;
@@ -1942,10 +1937,6 @@ bool LayerTreeHostImpl::HaveRootScrollLayer() const {
   return !!InnerViewportScrollLayer();
 }
 
-LayerImpl* LayerTreeHostImpl::RootLayer() const {
-  return active_tree_->root_layer();
-}
-
 LayerImpl* LayerTreeHostImpl::InnerViewportScrollLayer() const {
   return active_tree_->InnerViewportScrollLayer();
 }
@@ -2000,7 +1991,7 @@ void LayerTreeHostImpl::ActivateSyncTree() {
     pending_tree_->ProcessUIResourceRequestQueue();
 
     if (pending_tree_->needs_full_tree_sync()) {
-      TreeSynchronizer::SynchronizeTrees(pending_tree_->root_layer(),
+      TreeSynchronizer::SynchronizeTrees(pending_tree_.get(),
                                          active_tree_.get());
     }
 
@@ -2020,7 +2011,7 @@ void LayerTreeHostImpl::ActivateSyncTree() {
 
     TreeSynchronizer::PushLayerProperties(pending_tree(), active_tree());
     pending_tree_->PushPropertiesTo(active_tree_.get());
-    if (pending_tree_->root_layer())
+    if (!pending_tree_->LayerListIsEmpty())
       pending_tree_->property_trees()->ResetAllChangeTracking();
 
     // Now that we've synced everything from the pending tree to the active
@@ -3368,19 +3359,18 @@ LayerTreeHostImpl::ProcessLayerTreeMutations() {
 }
 
 static void CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
-                                LayerImpl* root_layer) {
-  if (!root_layer)
+                                LayerTreeImpl* tree_impl) {
+  if (tree_impl->LayerListIsEmpty())
     return;
 
-  return root_layer->layer_tree_impl()
-      ->property_trees()
-      ->scroll_tree.CollectScrollDeltas(scroll_info);
+  return tree_impl->property_trees()->scroll_tree.CollectScrollDeltas(
+      scroll_info);
 }
 
 std::unique_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {
   std::unique_ptr<ScrollAndScaleSet> scroll_info(new ScrollAndScaleSet());
 
-  CollectScrollDeltas(scroll_info.get(), active_tree_->root_layer());
+  CollectScrollDeltas(scroll_info.get(), active_tree_.get());
   scroll_info->page_scale_delta =
       active_tree_->page_scale_factor()->PullDeltaForMainThread();
   scroll_info->top_controls_delta =