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

cc/switches.cc

 // Copyright (c) 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.
 
 #include "cc/switches.h"
 
 namespace cc {
 namespace switches {
 
 // On platforms where checkerboards are used, prefer background colors instead
 // of checkerboards.
 const char kBackgroundColorInsteadOfCheckerboard[] =
     "background-color-instead-of-checkerboard";
 
 const char kDisableThreadedAnimation[]      = "disable-threaded-animation";
 
 const char kEnablePartialSwap[]             = "enable-partial-swap";
 
 const char kEnablePerTilePainting[]         = "enable-per-tile-painting";
 
 // Enables an alternative pinch-zoom gesture support, via the threaded
 // compositor.
 const char kEnablePinchInCompositor[]       = "enable-pinch-in-compositor";
 
+// Paint content on the compositor thread instead of the main thread.
+const char kImplSidePainting[] = "impl-side-painting";
+
 // When threaded compositing is turned on, wait until the entire frame has
 // content (i.e. jank) instead of showing checkerboards for missing content.
 const char kJankInsteadOfCheckerboard[] = "jank-instead-of-checkerboard";
 
-// Paint content on the compositor thread instead of the main thread.
-const char kImplSidePainting[] = "impl-side-painting";
+// Show metrics about overdraw in about:tracing recordings, such as the number
+// of pixels culled, and the number of pixels drawn, for each frame.
+const char kTraceOverdraw[] = "trace-overdraw";
 
 }  // namespace switches
 }  // namespace cc