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

cc/trees/single_thread_proxy.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/single_thread_proxy.h"
 
 #include "base/auto_reset.h"
 #include "base/debug/trace_event.h"
 #include "cc/base/thread.h"
 #include "cc/output/context_provider.h"
@@ skipping 17 matching lines @@
       output_surface_lost_(false),
       created_offscreen_context_provider_(false),
       renderer_initialized_(false),
       next_frame_is_newly_committed_frame_(false),
       inside_draw_(false) {
   TRACE_EVENT0("cc", "SingleThreadProxy::SingleThreadProxy");
   DCHECK(Proxy::IsMainThread());
   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() {
   DebugScopedSetImplThread impl(this);
   layer_tree_host_impl_ = layer_tree_host_->CreateLayerTreeHostImpl(this);
 }
 
 SingleThreadProxy::~SingleThreadProxy() {
   TRACE_EVENT0("cc", "SingleThreadProxy::~SingleThreadProxy");
   DCHECK(Proxy::IsMainThread());
@@ skipping 419 matching lines @@
 
 bool SingleThreadProxy::CommitPendingForTesting() { return false; }
 
 skia::RefPtr<SkPicture> SingleThreadProxy::CapturePicture() {
   // Impl-side painting only.
   NOTREACHED();
   return skia::RefPtr<SkPicture>();
 }
 
 }  // namespace cc