TextAutosizer: unlock multipliers on DOMContentLoaded.

This patch unlocks the text autosizing multipliers when
the document has finished parsing. This is necessary
to avoid weird effects when layout changes significantly
during page load. Resetting multipliers allows for a fresh
autosizing pass to take into account the whole document.

In particular this patch fixes issues with overlines on
http://forum.xda-developers.com and partially addresses
inconsistent font size on www.drudgereport.com
(see referenced bugs for details).

BUG=346786,131566

Review URL: https://codereview.chromium.org/189113014

git-svn-id: svn://svn.chromium.org/blink/trunk@169572 bbb929c8-8fbe-4397-9dbb-9b2b20218538