cc: Clarify text for single thread + impl-painting CHECK

cc/trees/single_thread_proxy.cc

Index: cc/trees/single_thread_proxy.cc
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index 31a7054b65436d5f4be58cc560d48338b846ad6f..87b38d500a6f7f5a1d8b81207bbdb8f70f920fc1 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -35,7 +35,8 @@ SingleThreadProxy::SingleThreadProxy(LayerTreeHost* layer_tree_host)
   DCHECK(layer_tree_host);
 
   // Impl-side painting not supported without threaded compositing.
-  CHECK(!layer_tree_host->settings().impl_side_painting);
+  CHECK(!layer_tree_host->settings().impl_side_painting)
+      << "Threaded compositing must be enabled to use impl-side painting.";
 }
 
 void SingleThreadProxy::Start() {