[FastTextAutosizer] Mark renderers as needing layout in applyMultiplier

This patch fixes a bug where renderers not marked for layout would
ignore the text autosizing multiplier change. This can occur when
text is appended to a block with existing children.

For children already marked as needing layout, this will have no
effect. I did a cursory review of some wikipedia articles and
forum comments and saw no regressions.

Before this patch: http://pr.gg/359796before.png
After this patch: http://pr.gg/359796after.png

BUG=359796

BUG=359796

Review URL: https://codereview.chromium.org/251383007

git-svn-id: svn://svn.chromium.org/blink/trunk@172585 bbb929c8-8fbe-4397-9dbb-9b2b20218538