Set break-word on figcaption

Set break-word on figcaption

This is using the non-standard value of word-break, instead of
overflow-wrap/word-wrap, since the latter doesn't produce the desired
effect.  That may be due to figure being displayed as a table, without
the layout being "fixed".

Unfortunately, non-Webkit based browsers don't implement this and
break-all is a little heavy handed for the few cases this turns up in
practice.

Bug: T171761
Change-Id: If123d30addb4c707ebed63bafd18dccba9afe158