Fix a placeholder layout bug in a case of nested flexible boxes and relative/absolute positions.

Whenever innerTextRenderer needs layout, a placeholder renderer should be laid out too.

BUG=361014, 362892
TEST=automated
R=esprehn, pfeldman

Review URL: https://codereview.chromium.org/242973005

git-svn-id: svn://svn.chromium.org/blink/trunk@172026 bbb929c8-8fbe-4397-9dbb-9b2b20218538