Avoid line-clamp style adjustment for displays that aren't block/inline.

Differential Revision: https://phabricator.services.mozilla.com/D319194

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2063759
gecko-commit: 1f8f01ab230fb9b04f0174429c4d56f7ec3ce8e9
gecko-commit-git: bc6b2cdc1eecb19aa6fefc14b4e37846e9f0ba01
gecko-reviewers: firefox-style-system-reviewers, emilio, layout-reviewers