[FastTextAutosizer] Fix footer autosizing on common sites.

This patch fixes a bug where footers were autosized incorrectly on many
common sites (e.g., 360.cn). The issue was that descendant clusters were
included in textLength, but they should not be.

With this patch we pass one additional test! I've also included a test
from 360.cn that failed before this patch.

BUG=302005

Review URL: https://codereview.chromium.org/176983002

git-svn-id: svn://svn.chromium.org/blink/trunk@167760 bbb929c8-8fbe-4397-9dbb-9b2b20218538