cc: Make single-thread renderer compositor always synchronous.

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 #include <map>
 #include <set>
@@ skipping 2100 matching lines @@
 
   if (GetRendererCapabilities().using_image) {
     unsigned image_target = settings_.use_image_texture_target;
     DCHECK_IMPLIES(
         image_target == GL_TEXTURE_RECTANGLE_ARB,
         context_provider->ContextCapabilities().gpu.texture_rectangle);
     DCHECK_IMPLIES(
         image_target == GL_TEXTURE_EXTERNAL_OES,
         context_provider->ContextCapabilities().gpu.egl_image_external);
 
-    if (settings_.use_zero_copy || IsSynchronousSingleThreaded()) {
+    if (settings_.use_zero_copy) {
       *resource_pool =
           ResourcePool::Create(resource_provider_.get(), image_target);
 
       *tile_task_worker_pool = ZeroCopyTileTaskWorkerPool::Create(
           task_runner, task_graph_runner, resource_provider_.get());
       return;
     }
 
     if (settings_.use_one_copy) {
+      // Synchronous single-threaded mode depends on tiles being ready to
+      // draw when raster is complete.  Therefore, it must use one of zero
+      // copy, software raster, or GPU raster.
+      DCHECK(!IsSynchronousSingleThreaded());
+
       // We need to create a staging resource pool when using copy rasterizer.
       *staging_resource_pool =
           ResourcePool::Create(resource_provider_.get(), image_target);
       *resource_pool =
           ResourcePool::Create(resource_provider_.get(), GL_TEXTURE_2D);
 
       *tile_task_worker_pool = OneCopyTileTaskWorkerPool::Create(
           task_runner, task_graph_runner, context_provider,
           resource_provider_.get(), staging_resource_pool_.get());
       return;
@@ skipping 1295 matching lines @@
   new_target.SetToMin(layer_impl->MaxScrollOffset());
 
   curve->UpdateTarget(
       animation->TrimTimeToCurrentIteration(CurrentBeginFrameArgs().frame_time)
           .InSecondsF(),
       new_target);
 
   return true;
 }
 }  // namespace cc