Omit empty TOC ("Contents" heading and empty OL) if there are no items in the TOC.

With the old DCC, the empty TOC only appears as a somewhat mysterious gray rectangle, but with the new DCC, we have a contents header, which makes this problem more obvious.

BUG=341696

To reproduce: 

1. Load a page with no headings (or headings that don't appear in the TOC), such as /apps/publish_app.html
2. Without the patch, an empty Contents heading appears.
3. With the patch, the contents doesn't appear (technically, there's still a section, but no visible text).

Review URL: https://codereview.chromium.org/140733009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249897 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249897