style: Remove `max-height` from the `input` style.

Some pages, like Google, want to set height explicitly.
auto merge of #3602 : pcwalton/servo/max-height-input, r=jdm

Some pages, like Google, want to set height explicitly.

r? @jdm
layout: Mark the styles of elements with pseudos as unshareable.

Makes multiple `<br>` elements work, since those are implemented via
`before` pseudos.