Internals should always cause a layout before calling into TextIterator

Calling into TextIterator without causing a layout first doesn't work and will break
once we lazy attach everywhere. This fixes lots of tests that break once we lazy attach
inside the parser.

Review URL: https://chromiumcodereview.appspot.com/22978019

git-svn-id: svn://svn.chromium.org/blink/trunk@156530 bbb929c8-8fbe-4397-9dbb-9b2b20218538