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

R=danakj@chromium.org
BUG=231458

Review URL: https://codereview.chromium.org/14269002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194606 0039d316-1c4b-4281-b951-d872f2087c98

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() {