content: Add switch to enable checker-imaging.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 6588d8fad9ce25c949d8bff060121201cd0dc792..99543ccfc3ae51905a5aebca2e4c5caa05080b8b 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -73,6 +73,10 @@ class CC_EXPORT LayerTreeHost : public NON_EXPORTED_BASE(SurfaceReferenceOwner),
     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();