[MD Bookmarks] Fix segfault when calling chrome.bookmarks.remove().

chrome/browser/sync/glue/extensions_activity_monitor.cc

 // Copyright 2013 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/sync/glue/extensions_activity_monitor.h"
 
 #include "components/sync/base/extensions_activity.h"
 #include "content/public/browser/browser_thread.h"
 #include "extensions/features/features.h"
 
@@ skipping 29 matching lines @@
 
 void ExtensionsActivityMonitor::Observe(
     int type,
     const content::NotificationSource& source,
     const content::NotificationDetails& details) {
 #if BUILDFLAG(ENABLE_EXTENSIONS)
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   DCHECK_EQ(extensions::NOTIFICATION_EXTENSION_BOOKMARKS_API_INVOKED, type);
   const extensions::Extension* extension =
       content::Source<const extensions::Extension>(source).ptr();
+  if (!extension)
+    return;
+
   const extensions::BookmarksFunction* f =
       content::Details<const extensions::BookmarksFunction>(details).ptr();
   switch (f->histogram_value()) {
     case extensions::functions::BOOKMARKS_UPDATE:
     case extensions::functions::BOOKMARKS_MOVE:
     case extensions::functions::BOOKMARKS_CREATE:
     case extensions::functions::BOOKMARKS_REMOVETREE:
     case extensions::functions::BOOKMARKS_REMOVE:
       extensions_activity_->UpdateRecord(extension->id());
       break;
     default:
       break;
   }
 #endif
 }
 
 const scoped_refptr<syncer::ExtensionsActivity>&
 ExtensionsActivityMonitor::GetExtensionsActivity() {
   return extensions_activity_;
 }
 
 }  // namespace browser_sync