Text Autosizer: consistently autosize phpBB forum posts.

Traditionally Text Autosizer has issues with forum comments.
This patch addresses the issue on fora powered by phpBB forum
generator software. The solution is similar to vBulletin
(https://codereview.chromium.org/23020026/).

In this approach we define two funtions:
1. one for detecting the content type, e.g. phpBB,
2. one for detecting the blocks containing the posts by the users.

The idea is based on the fact that forum pages have certain
signatures. This method has the advantage that the overhead
for the non-forum websites is minimal.

BUG=312880

Review URL: https://codereview.chromium.org/47493003

git-svn-id: svn://svn.chromium.org/blink/trunk@161357 bbb929c8-8fbe-4397-9dbb-9b2b20218538