Make root layer DCHECK in RWC::layout less general

content/renderer/gpu/render_widget_compositor.cc

 // Copyright (c) 2013 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 "content/renderer/gpu/render_widget_compositor.h"
 
 #include <limits>
 #include <string>
 
 #include "base/command_line.h"
@@ skipping 768 matching lines @@
   double deadline_sec = (args.deadline - base::TimeTicks()).InSecondsF();
   double interval_sec = args.interval.InSecondsF();
   WebBeginFrameArgs web_begin_frame_args =
       WebBeginFrameArgs(frame_time_sec, deadline_sec, interval_sec);
   widget_->webwidget()->beginFrame(web_begin_frame_args);
 }
 
 void RenderWidgetCompositor::Layout() {
   widget_->webwidget()->layout();
 
-  DCHECK(layer_tree_host_->root_layer());
   if (temporary_copy_output_request_) {
+    DCHECK(layer_tree_host_->root_layer());
     layer_tree_host_->root_layer()->RequestCopyOfOutput(
         temporary_copy_output_request_.Pass());
   }
 }
 
 void RenderWidgetCompositor::ApplyViewportDeltas(
     const gfx::Vector2d& inner_delta,
     const gfx::Vector2d& outer_delta,
     float page_scale,
     float top_controls_delta) {
@@ skipping 67 matching lines @@
   widget_->OnSwapBuffersAborted();
 }
 
 void RenderWidgetCompositor::RateLimitSharedMainThreadContext() {
   cc::ContextProvider* provider =
       RenderThreadImpl::current()->SharedMainThreadContextProvider().get();
   provider->ContextGL()->RateLimitOffscreenContextCHROMIUM();
 }
 
 }  // namespace content