Fix a crash on closing tab while the plugin settings bubble is visible

chrome/browser/ui/content_settings/content_setting_bubble_model.cc

Index: chrome/browser/ui/content_settings/content_setting_bubble_model.cc
diff --git a/chrome/browser/ui/content_settings/content_setting_bubble_model.cc b/chrome/browser/ui/content_settings/content_setting_bubble_model.cc
index 1e3433a5ca01d000f1854756f0ec8aa07225a17a..d8bd45b02e6701647b234967d1eb9b92822c3340 100644
--- a/chrome/browser/ui/content_settings/content_setting_bubble_model.cc
+++ b/chrome/browser/ui/content_settings/content_setting_bubble_model.cc
@@ -497,7 +497,10 @@ ContentSettingPluginBubbleModel::~ContentSettingPluginBubbleModel() {
 
 void ContentSettingPluginBubbleModel::OnCustomLinkClicked() {
   content::RecordAction(UserMetricsAction("ClickToPlay_LoadAll_Bubble"));
-  DCHECK(web_contents());
+  // Web contents can be NULL if the tab was closed while the plugins
+  // settings bubble is visible.
+  if (!web_contents())
+    return;
 #if defined(ENABLE_PLUGINS)
   // TODO(bauerb): We should send the identifiers of blocked plug-ins here.
   ChromePluginServiceFilter::GetInstance()->AuthorizeAllPlugins(