Text Autosizer: ancestry-based hashing approach.

Initial implementation of the ancestry-based hashing approach
for autosizing repeating patterns, like forum comments.

For each cluster we compute a hash value taking into account
ancestor tag names and style properties. These hashes are stored
in a HashSet and used to consistently autosize clusters with the
same hash value.

To improve efficiency the hashes of the ancestors are cached,
so they need to be computed only once per call to processSubtree().

Testing on forum sites, e.g.
- http://forum.vudu.com/showthread.php?t=202551 (vBulletin)
- https://www.phpbb.com/community/viewtopic.php?f=46&t=2203461 (phpBB)
- http://bitcointalk.org/ (SMF)
shows good results, i.e. the comments are autosized consistently.

BUG=135947,312880
NOTRY=true

Review URL: https://codereview.chromium.org/65643011

git-svn-id: svn://svn.chromium.org/blink/trunk@164240 bbb929c8-8fbe-4397-9dbb-9b2b20218538