Mac: Correct CALayer reuse logic

ui/accelerated_widget_mac/ca_renderer_layer_tree.mm

 // Copyright 2016 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 "ui/accelerated_widget_mac/ca_renderer_layer_tree.h"
 
 #include <AVFoundation/AVFoundation.h>
 #include <CoreMedia/CoreMedia.h>
 #include <CoreVideo/CoreVideo.h>
 #include <GLES2/gl2extchromium.h>
@@ skipping 538 matching lines @@
     update_ca_filter = old_layer->ca_filter != ca_filter;
   } else {
     if (use_av_layer) {
       av_layer.reset([[AVSampleBufferDisplayLayer alloc] init]);
       ca_layer.reset([av_layer retain]);
       [av_layer setVideoGravity:AVLayerVideoGravityResize];
     } else {
       ca_layer.reset([[CALayer alloc] init]);
     }
     [ca_layer setAnchorPoint:CGPointZero];
-    [superlayer addSublayer:ca_layer];
+    if (old_layer && old_layer->ca_layer)
+      [superlayer replaceSublayer:old_layer->ca_layer with:ca_layer];
+    else
+      [superlayer addSublayer:ca_layer];
   }
   DCHECK_EQ([ca_layer superlayer], superlayer);
   bool update_anything = update_contents || update_contents_rect ||
                          update_rect || update_background_color ||
                          update_ca_edge_aa_mask || update_opacity ||
                          update_ca_filter;
   if (use_av_layer) {
     if (update_contents) {
       if (cv_pixel_buffer) {
         AVSampleBufferDisplayLayerEnqueueCVPixelBuffer(av_layer,
@@ skipping 52 matching lines @@
     } else {
       // Grey represents a CALayer that has not changed.
       color.reset(CGColorCreateGenericRGB(0.5, 0.5, 0.5, 1));
     }
     [ca_layer setBorderWidth:1];
     [ca_layer setBorderColor:color];
   }
 }
 
 }  // namespace ui