Bug 1092007 part 1: Adjust some in-content CSS to account for flex definite-sizing change. r=johannh

As of this bug, flex items in a vertical flex container will sometimes be
considered to have "indefinite" sizes, i.e. percent sizes in them will no
longer resolve. To work around this, they need to provide a definite flex-basis
(e.g. as part of the "flex" shorthand property) if we want percent sizes to
resolve (instead of being treated as "auto") inside of them.

Differential Revision: https://phabricator.services.mozilla.com/D44734

--HG--
extra : moz-landing-system : lando

toolkit/themes/shared/in-content/info-pages.inc.css

@@ -116,7 +116,7 @@ button {
 /* Trees */
 .tree-container {
   margin-top: 1.2em;
-  flex-grow: 1;
+  flex: 1 0px;
   min-height: 12em;
 }