content: Add switch to enable checker-imaging.

cc/trees/layer_tree_host.h

 // 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.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_H_
 #define CC_TREES_LAYER_TREE_HOST_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 55 matching lines @@
                                 public NON_EXPORTED_BASE(MutatorHostClient) {
  public:
   // TODO(sad): InitParams should be a movable type so that it can be
   // std::move()d to the Create* functions.
   struct CC_EXPORT InitParams {
     LayerTreeHostClient* client = nullptr;
     TaskGraphRunner* task_graph_runner = nullptr;
     LayerTreeSettings const* settings = nullptr;
     scoped_refptr<base::SingleThreadTaskRunner> main_task_runner;
     MutatorHost* mutator_host = nullptr;
+
+    // The image worker task runner is used to schedule image decodes. The
+    // compositor thread may make sync calls to this thread, analogous to the
+    // raster worker threads.
     scoped_refptr<base::SequencedTaskRunner> image_worker_task_runner;
 
     InitParams();
     ~InitParams();
   };
 
   static std::unique_ptr<LayerTreeHost> CreateThreaded(
       scoped_refptr<base::SingleThreadTaskRunner> impl_task_runner,
       InitParams* params);
 
@@ skipping 512 matching lines @@
   bool in_paint_layer_contents_ = false;
 
   MutatorHost* mutator_host_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHost);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_H_