Guard access to |internal_plugins_| in PluginsList::ShouldLoadPluginUsingPluginList() with a lock.

BUG=297098

Review URL: https://codereview.chromium.org/25269005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226619 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226619