Consistently autosize vBulletin forum comments.

vBulletin forum comments typically use <div id="post_message_*"> to define forum posts. This CL exploits this information and treats all text inside such a "post_message_*" div as autosizable.

This resolves the issue with short comments which do not get autosized by default and often result in annoying layouts with posts having very different font sizes. For efficiency the forum-specific detection code only runs when <meta name="generator" content="vBulletin*"> tag is present.

The described approach does not work for all forums though, so a more elaborate algorithm is required to detect forum comments in general possibly involving extraction of repeating patterns from the DOM tree.

BUG=135947

Review URL: https://chromiumcodereview.appspot.com/23020026

git-svn-id: svn://svn.chromium.org/blink/trunk@157789 bbb929c8-8fbe-4397-9dbb-9b2b20218538