cc: Guard overdraw metrics behind the --trace-overdraw command-line flag.

cc/settings.h

 // Copyright 2012 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.
 
 #ifndef CC_SETTINGS_H_
 #define CC_SETTINGS_H_
 
 #include "cc/cc_export.h"
 
 namespace cc {
 
 // This file is for settings that apply to all compositors.  Add settings to
 // LayerTreeSettings if a ui and renderer compositor might not want the same
 // setting.
 
 class CC_EXPORT Settings {
 public:
     static bool perTilePaintingEnabled();
     static bool partialSwapEnabled();
     static bool acceleratedAnimationEnabled();
     static bool pageScalePinchZoomEnabled();
     static bool jankInsteadOfCheckerboard();
     static bool backgroundColorInsteadOfCheckerboard();
+    static bool traceOverdraw();
 
     static void setPartialSwapEnabled(bool);
     static void setPerTilePaintingEnabled(bool);
     static void setAcceleratedAnimationEnabled(bool);
     static void setPageScalePinchZoomEnabled(bool);
 
     static void resetForTest();
 };
 
 } // namespace cc
 
 #endif  // CC_SETTINGS_H_