Fix directory traversal in extension_resources.cc. Adds test case, which is complicated by several other pre-existing checks.

      BUG=161836

Review URL: https://chromiumcodereview.appspot.com/11308204

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@169949 0039d316-1c4b-4281-b951-d872f2087c98