Move computeAutosizedFontSize into FastTextAutosizer.

Also update comment with link to current documentation.

BUG=385380

Review URL: https://codereview.chromium.org/348933004

git-svn-id: svn://svn.chromium.org/blink/trunk@176699 bbb929c8-8fbe-4397-9dbb-9b2b20218538