Fix a regression that was introduced in http://codereview.chromium.org/6010004/

chrome/browser/tab_contents/thumbnail_generator.cc

 // Copyright (c) 2009 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 "chrome/browser/tab_contents/thumbnail_generator.h"
 
 #include <algorithm>
 #include <map>
 
 #include "base/metrics/histogram.h"
@@ skipping 162 matching lines @@
 }
 
 void ThumbnailGenerator::MonitorRenderer(RenderWidgetHost* renderer,
                                          bool monitor) {
   Source<RenderWidgetHost> renderer_source = Source<RenderWidgetHost>(renderer);
   bool currently_monitored =
       registrar_.IsRegistered(
         this,
         NotificationType::RENDER_WIDGET_HOST_WILL_DESTROY_BACKING_STORE,
         renderer_source);
-  if (monitor |= currently_monitored) {
+  if (monitor != currently_monitored) {
     if (monitor) {
       registrar_.Add(
           this,
           NotificationType::RENDER_WIDGET_HOST_WILL_DESTROY_BACKING_STORE,
           renderer_source);
       registrar_.Add(
           this,
           NotificationType::RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE,
           renderer_source);
       registrar_.Add(
@@ skipping 333 matching lines @@
                  &ThumbnailGenerator::ShownDelayHandler);
   }
 }
 
 void ThumbnailGenerator::EraseHostFromShownList(RenderWidgetHost* widget) {
   std::vector<RenderWidgetHost*>::iterator found =
       std::find(shown_hosts_.begin(), shown_hosts_.end(), widget);
   if (found != shown_hosts_.end())
     shown_hosts_.erase(found);
 }