Add debug method to print surface references.

cc/surfaces/surface.cc

 // Copyright 2014 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/surfaces/surface.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 98 matching lines @@
     for (const auto& render_pass : current_frame_->render_pass_list) {
       for (auto& request : render_pass->copy_requests) {
         copy_requests->insert(
             std::make_pair(render_pass->id, std::move(request)));
       }
       render_pass->copy_requests.clear();
     }
   }
 }
 
-const CompositorFrame& Surface::GetEligibleFrame() {
+const CompositorFrame& Surface::GetEligibleFrame() const {
   DCHECK(current_frame_);
   return current_frame_.value();
 }
 
 void Surface::TakeLatencyInfo(std::vector<ui::LatencyInfo>* latency_info) {
   if (!current_frame_)
     return;
   if (latency_info->empty()) {
     current_frame_->metadata.latency_info.swap(*latency_info);
     return;
@@ skipping 41 matching lines @@
 void Surface::ClearCopyRequests() {
   if (current_frame_) {
     for (const auto& render_pass : current_frame_->render_pass_list) {
       for (const auto& copy_request : render_pass->copy_requests)
         copy_request->SendEmptyResult();
     }
   }
 }
 
 }  // namespace cc