Make TextIterator constructor to update layout

This patch makes TextIterator constructor to call update layout to avoid
each call sites to update layout for ease of code maintenance.

BUG=392350
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/379743002

git-svn-id: svn://svn.chromium.org/blink/trunk@177809 bbb929c8-8fbe-4397-9dbb-9b2b20218538