Make the two-argument createElement check Custom Elements are enabled.

BUG=304807
TEST=virtual/stable/webexposed/custom-elements.html

Review URL: https://codereview.chromium.org/26654002

git-svn-id: svn://svn.chromium.org/blink/trunk@159221 bbb929c8-8fbe-4397-9dbb-9b2b20218538