Delete background tab IOSurfaces on Mac.

content/browser/renderer_host/compositing_iosurface_mac.mm

 // 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 "content/browser/renderer_host/compositing_iosurface_mac.h"
 
 #include <OpenGL/OpenGL.h>
 #include <vector>
 
 #include "base/command_line.h"
@@ skipping 82 matching lines @@
   UnrefIOSurface();
 }
 
 void CompositingIOSurfaceMac::SetIOSurface(uint64 io_surface_handle) {
   CGLSetCurrentContext(cglContext_);
   MapIOSurfaceToTexture(io_surface_handle);
   CGLSetCurrentContext(0);
 }
 
 void CompositingIOSurfaceMac::DrawIOSurface(NSView* view) {
-  TRACE_EVENT0("browser", "CompositingIOSurfaceMac::DrawIOSurface");
   CGLSetCurrentContext(cglContext_);
 
   bool has_io_surface = MapIOSurfaceToTexture(io_surface_handle_);
 
+  TRACE_EVENT1("browser", "CompositingIOSurfaceMac::DrawIOSurface",
+               "has_io_surface", has_io_surface);
+
   [glContext_ setView:view];
   NSSize window_size = [view frame].size;
   glViewport(0, 0, window_size.width, window_size.height);
 
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   glOrtho(0, window_size.width, window_size.height, 0, -1, 1);
   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();
 
@@ skipping 166 matching lines @@
   glDisableClientState(GL_TEXTURE_COORD_ARRAY);
 }
 
 void CompositingIOSurfaceMac::UnrefIOSurfaceWithContextCurrent() {
   if (texture_) {
     glDeleteTextures(1, &texture_);
     texture_ = 0;
   }
 
   io_surface_.reset();
+
+  // Forget the ID, because even if it is still around when we want to use it
+  // again, OSX may have reused the same ID for a new tab and we don't want to
+  // blit random tab contents.
+  io_surface_handle_ = 0;
 }
 
 void CompositingIOSurfaceMac::GlobalFrameDidChange() {
   [glContext_ update];
 }
 
 void CompositingIOSurfaceMac::ClearDrawable() {
   [glContext_ clearDrawable];
   UnrefIOSurface();
 }