layout: Don't delete non-text boxes from the list of input boxes when
flushing text clumps. Fixes this page:

    http://en.wikipedia.org/wiki/Yellow_River

This is not the most efficient thing to do; a FIXME has been added
noting what is most correct.
auto merge of #1381 : pcwalton/servo/bogus-box-deletion, r=jdm

flushing text clumps. Fixes this page:

    http://en.wikipedia.org/wiki/Yellow_River

This is not the most efficient thing to do; a FIXME has been added
noting what is most correct.

r? @jdm