layout: Make non-inline `line-height` specify the minimum line height

for its inline kids per CSS 2.1 § 10.8.1.

Needed for Acid2 line 12.

This adds the infrastructure needed for derived pseudo-properties, which
are a useful way to propagate things down during the cascade. This is
similar to WebKit's `-webkit-text-decorations-in-effect`. Eventually I
would like to replace `FlowFlagsInfo` with this infrastructure.

r? @SimonSapin